Showing how to test a basic insert trigger in Apex

LeadTrigger.trigger

//Deploy with: sfdx force:source:deploy -p force-app/main/default
//Note: 2 or more -p arguments don't appear to work.  It only deploys the path in the last argument.
trigger LeadTrigger on Lead (before insert) {
  for(Lead leadReference : trigger.new) {
    leadReference.Age_Nearest__c = 1;
  }
}

TestLead.cls

//TestLead.cls
//Run with: sfdx force:apex:test:run -n TestLead --resultformat=human
@isTest
public class TestLead {
    @isTest
    public static void leadShouldInsert() {
        Lead subject = makeLead();

        Test.startTest();
        Database.SaveResult result = Database.insert(subject);
        Test.stopTest();

        System.assert(result.isSuccess());
    }

    @isTest
    public static void createLeadWhenTodayIsBirthday_AgeNearestIsTodaysAge() {
        Date aYearAgo = Date.today().addYears(-1);
        Lead subject = makeLead(aYearAgo);

        Test.startTest();
        Database.insert(subject);
        Lead fetched = [select id, Age_Nearest__c from Lead where id = :subject.id];
        Test.stopTest();

        System.assertEquals(1, fetched.Age_Nearest__c);
    }

    private static Lead makeLead() {
        Lead lead = new Lead();
        lead.Birthdate__c = Date.newInstance(2000, 1, 1);
        lead.Company = 'Initech';
        lead.LastName = 'MacReady';

        return lead;
    }

    private static Lead makeLead(Date birthdate) {
        Lead lead = new Lead();
        lead.Birthdate__c = birthdate;
        lead.Company = 'Initech';
        lead.LastName = 'MacReady';

        return lead;
    }
}