Apex Test Classes using System.Runas

gistfile1.txt

// Example One - Create the User and run tests under that user
Profile p = [select id from profile where name='System Administrator'];
User u = new User(
    alias = 'abc123xyz', 
    email='unit.test@email123.com',
    emailencodingkey='UTF-8',
    firstname='Sam',
    lastname='Franks', 
    languagelocalekey='en_US',
    localesidkey='en_GB', 
    profileid = p.Id,
    timezonesidkey='Europe/London', 
    username='unit.test@email123.com'
);
insert u;

System.runAs(u){
    Opportunity newOpp = new Opportunity(
        Name = 'Test Opp',
        Account = a.Id,
        CloseDate = Date.Today() + 90,
        Primary_Contact__c = c.Id,
        StageName = 'Prospect',
        Amount = 1.00,
        Product_Family__c = 'Heavy Metal',
        //RecordTypeId = '01244000001MMcz', <-- This is how you can statically set the record type or use a variable
        Pricebook2 = pb
    );
    insert newOpp;
}

// Method to run the tests as the user executing the tests
User thisUser = [SELECT Id FROM User WHERE Id = :UserInfo.getUserId()];
System.runAs(thisUser){
  // Test Go Here
}