https://github.com/ethereum/remix-ide/blob/e32108c838c7a47fab60ac110a174f5b3d053706/docs/unittesting_examples.md (Testing by Example - Remix)

AttendanceRegister.sol

pragma solidity >=0.4.22 <0.7.0;
contract AttendanceRegister {
    struct Student{
            string name;
            uint class;
        }

    event Added(string name, uint class, uint time);

    mapping(uint => Student) public register; // roll number => student details

    function add(uint rollNumber, string memory name, uint class) public returns (uint256){
        require(class > 0 && class <= 12, "Invalid class");
        require(register[rollNumber].class == 0, "Roll number not available");
        Student memory s = Student(name, class);
        register[rollNumber] = s;
        emit Added(name, class, now);
        return rollNumber;
    }
    
    function getStudentName(uint rollNumber) public view returns (string memory) {
        return register[rollNumber].name;
    }
}

AttendanceRegister_test.sol

pragma solidity >=0.4.22 <0.7.0;
import "remix_tests.sol"; // this import is automatically injected by Remix.
import "./AttendanceRegister.sol";

contract AttendanceRegisterTest {
   
    AttendanceRegister ar;
    
    /// 'beforeAll' runs before all other tests
    function beforeAll () public {
        // Create an instance of contract to be tested
        ar = new AttendanceRegister();
    }
    
    /// For solidity version greater or equal to 0.6.0, 
    /// See: https://solidity.readthedocs.io/en/v0.6.0/control-structures.html#try-catch
    /// Test 'add' using try-catch
    function testAddSuccessUsingTryCatch() public {
        // This will pass
        try ar.add(101, 'secondStudent', 11) returns (uint256 r) {
            Assert.equal(r, 101, 'wrong rollNumber');
        } catch Error(string memory /*reason*/) {
            // This is executed in case
            // revert was called inside getData
            // and a reason string was provided.
            Assert.ok(false, 'failed with reason');
        } catch (bytes memory /*lowLevelData*/) {
            // This is executed in case revert() was used
            // or there was a failing assertion, division
            // by zero, etc. inside getData.
            Assert.ok(false, 'failed unexpected');
        }
    }
    
    /// Test failure case of 'add' using try-catch
    function testAddFailureUsingTryCatch1() public {
        // This will revert on 'require(class > 0 && class <= 12, "Invalid class");' for class '13'
        try ar.add(101, 'secondStudent', 13) returns (uint256 r) {
            Assert.ok(false, 'method execution should fail');
        } catch Error(string memory reason) {
            // Compare failure reason, check if it is as expected
            Assert.equal(reason, 'Invalid class', 'failed with unexpected reason');
        } catch (bytes memory /*lowLevelData*/) {
            Assert.ok(false, 'failed unexpected');
        }
    }
    
    /// Test another failure case of 'add' using try-catch
    function testAddFailureUsingTryCatch2() public {
        // This will revert on 'require(register[rollNumber].class == 0, "Roll number not available");' for rollNumber '101'
        try ar.add(101, 'secondStudent', 11) returns (uint256 r) {
            Assert.ok(false, 'method execution should fail');
        } catch Error(string memory reason) {
            // Compare failure reason, check if it is as expected
            Assert.equal(reason, 'Roll number not available', 'failed with unexpected reason');
        } catch (bytes memory /*lowLevelData*/) {
            Assert.ok(false, 'failed unexpected');
        }
    }
    
    /// For solidity version less than 0.6.0, low level call can be used
    /// See: https://solidity.readthedocs.io/en/v0.6.0/units-and-global-variables.html#members-of-address-types
    /// Test success case of 'add' using low level call
    function testAddSuccessUsingCall() public {
        bytes memory methodSign = abi.encodeWithSignature('add(uint256,string,uint256)', 102, 'firstStudent', 10);
        (bool success, bytes memory data) = address(ar).call(methodSign);
        // 'success' stores the result in bool, this can be used to check whether method call was successful
        Assert.equal(success, true, 'execution should be successful');
        // 'data' stores the returned data which can be decoded to get the actual result
        uint rollNumber = abi.decode(data, (uint256));
        // check if result is as expected
        Assert.equal(rollNumber, 102, 'wrong rollNumber');
    }
    
    /// Test failure case of 'add' using low level call
    function testAddFailureUsingCall() public {
        bytes memory methodSign = abi.encodeWithSignature('add(uint256,string,uint256)', 102, 'duplicate', 10);
        (bool success, bytes memory data) = address(ar).call(methodSign);
        // 'success' will be false if method execution is not successful
        Assert.equal(success, false, 'execution should be successful');
    }
}

Donations.sol

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.4;

contract donations{ 
    struct Donation {
        uint id;
        uint amount;
        string donor;
        string message;
        uint timestamp; //seconds since unix start
    }
    uint amount = 0;
    uint id = 0;
    mapping(address => uint) public balances;
    mapping(address => Donation[]) public donationsMap;

    function donate(address _recipient, string memory _donor, string memory _msg) public payable {
        require(msg.value > 0, "The donation needs to be >0 in order for it to go through");
        amount = msg.value;
        balances[_recipient] += amount;        
        donationsMap[_recipient].push(Donation(id++,amount,_donor,_msg,block.timestamp));
    }

    function withdraw() public {  //whole thing by default.
        amount = balances[msg.sender];
        balances[msg.sender] -= amount;
        require(amount > 0, "Your current balance is 0");
        (bool success,) = msg.sender.call{value:amount}("");
        if(!success){
            revert();
        }
    }
  
    function balances_getter(address _recipient) public view returns (uint){
            return balances[_recipient];
    }
    
    function getBalance() public view returns(uint) {
            return msg.sender.balance;
    }
}

Donations_test.sol

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.4.22 <0.9.0;
import "remix_tests.sol"; 
import "remix_accounts.sol";
import "../donations.sol";

contract testSuite is donations {
    address acc0 = TestsAccounts.getAccount(0); //owner by default
    address acc1 = TestsAccounts.getAccount(1);
    address acc2 = TestsAccounts.getAccount(2);
    address acc3 = TestsAccounts.getAccount(3);
    address recipient = TestsAccounts.getAccount(4); //recipient

    /// #value: 1000000000000000000
    /// #sender: account-1
    function donateAcc1AndCheckBalance() public payable{
        Assert.equal(msg.value, 1000000000000000000, 'value should be 1 Eth');
        donate(recipient, "Mario", "Are you a bird?");
        Assert.equal(balances_getter(recipient), 1000000000000000000, 'balances should be 1 Eth');
    }
    
    /// #value: 1000000000000000000
    /// #sender: account-2
    function donateAcc2AndCheckBalance() public payable{
        Assert.equal(msg.value, 1000000000000000000, 'value should be 1 Eth');
        donate(recipient, "Tom", "Are you a plane?");
        Assert.equal(balances_getter(recipient), 2000000000000000000, 'balances should be 2 Eth');
    }
    
    /// #value: 2000000000000000000
    /// #sender: account-3
    function donateAcc3AndCheckBalance() public payable{
        Assert.equal(msg.value, 2000000000000000000, 'value should be 1 Eth');
        donate(recipient, "Maria", "Are you a car?");
        Assert.equal(balances_getter(recipient), 4000000000000000000, 'balances should be 4 Eth');
    }
    
    /// #sender: account-4
    function withdrawDonations() public payable{
        uint initialBal = getBalance();
        withdraw();
        uint finalBal = getBalance();
        Assert.equal(finalBal-initialBal, 4000000000000000000, 'balances should be 4 Eth');
    }
}

Sender.sol

pragma solidity >=0.4.22 <0.7.0;
contract Sender {
    address private owner;
    
    constructor() public {
        owner = msg.sender;
    }
    
    function updateOwner(address newOwner) public {
        require(msg.sender == owner, "only current owner can update owner");
        owner = newOwner;
    }
    
    function getOwner() public view returns (address) {
        return owner;
    }
}

Sender_test.sol

pragma solidity >=0.4.22 <0.7.0;
import "remix_tests.sol"; // this import is automatically injected by Remix
import "remix_accounts.sol";
import "./Sender.sol";

// Inherit 'Sender' contract
contract SenderTest is Sender {
    /// Define variables referring to different accounts
    address acc0;
    address acc1;
    address acc2;
    
    /// Initiate accounts variable
    function beforeAll() public {
        acc0 = TestsAccounts.getAccount(0); 
        acc1 = TestsAccounts.getAccount(1);
        acc2 = TestsAccounts.getAccount(2);
    }
    
    /// Test if initial owner is set correctly
    function testInitialOwner() public {
        // account at zero index (account-0) is default account, so current owner should be acc0
        Assert.equal(getOwner(), acc0, 'owner should be acc0');
    }
    
    /// Update owner first time
    /// This method will be called by default account(account-0) as there is no custom sender defined
    function updateOwnerOnce() public {
        // check method caller is as expected
        Assert.ok(msg.sender == acc0, 'caller should be default account i.e. acc0');
        // update owner address to acc1
        updateOwner(acc1);
        // check if owner is set to expected account
        Assert.equal(getOwner(), acc1, 'owner should be updated to acc1');
    }
    
    /// Update owner again by defining custom sender
    /// #sender: account-1 (sender is account at index '1')
    function updateOwnerOnceAgain() public {
        // check if caller is custom and is as expected
        Assert.ok(msg.sender == acc1, 'caller should be custom account i.e. acc1');
        // update owner address to acc2. This will be successful because acc1 is current owner & caller both
        updateOwner(acc2);
        // check if owner is set to expected account i.e. account2
        Assert.equal(getOwner(), acc2, 'owner should be updated to acc2');
    }
}

Simple_storage.sol

pragma solidity >=0.4.22 <0.7.0;

contract SimpleStorage {
    uint public storedData;

    constructor() public {
        storedData = 100;
    }

    function set(uint x) public {
        storedData = x;
    }

    function get() public view returns (uint retVal) {
        return storedData;
    }
}

simple_storage_test.sol

pragma solidity >=0.4.22 <0.7.0;
import "remix_tests.sol";
import "./Simple_storage.sol";

contract MyTest {
    SimpleStorage foo;

    // beforeEach works before running each test
    function beforeEach() public {
        foo = new SimpleStorage();
    }

    /// Test if initial value is set correctly
    function initialValueShouldBe100() public returns (bool) {
        return Assert.equal(foo.get(), 100, "initial value is not correct");
    }

    /// Test if value is set as expected
    function valueIsSet200() public returns (bool) {
        foo.set(200);
        return Assert.equal(foo.get(), 200, "value is not 200");
    }
}

Value.sol

pragma solidity >=0.4.22 <0.7.0;
contract Value {
    uint256 public tokenBalance;
    
    constructor() public {
        tokenBalance = 0;
    }
    
    function addValue() payable public {
        tokenBalance = tokenBalance + (msg.value/10);
    } 
    
    function getTokenBalance() view public returns (uint256) {
        return tokenBalance;
    }
}

Value_test.sol

pragma solidity >=0.4.22 <0.7.0;
import "remix_tests.sol"; 
import "./Value.sol";

contract ValueTest{
    Value v;
    
    function beforeAll() public {
        // create a new instance of Value contract
        v = new Value();
    }
    
    /// Test initial balance
    function testInitialBalance() public {
        // initially token balance should be 0
        Assert.equal(v.getTokenBalance(), 0, 'token balance should be 0 initially');
    }
    
    /// For Solidity version greater than 0.6.1
    /// Test 'addValue' execution by passing custom ether amount 
    /// #value: 200
    function addValueOnce() public payable {
        // check if value is same as provided through devdoc
        Assert.equal(msg.value, 200, 'value should be 200');
        // execute 'addValue'
        v.addValue{gas: 40000, value: 200}(); // introduced in Solidity version 0.6.2
        // As per the calculation, check the total balance
        Assert.equal(v.getTokenBalance(), 20, 'token balance should be 20');
    }
    
    /// For Solidity version less than 0.6.2
    /// Test 'addValue' execution by passing custom ether amount again using low level call
    /// #value: 100
    function addValueAgain() public payable {
        Assert.equal(msg.value, 100, 'value should be 100');
        bytes memory methodSign = abi.encodeWithSignature('addValue()');
        (bool success, bytes memory data) = address(v).call.gas(40000).value(100)(methodSign);
        Assert.equal(success, true, 'execution should be successful');
        Assert.equal(v.getTokenBalance(), 30, 'token balance should be 30');
    }
}