public class OpportunityHandler {
    public static void checkOppForAttachment (List<Opportunity> oppList){
        list<Id> oppIds = new List<Id>();     
        for(Opportunity opp : oppList){
            if(opp.stageName == 'Closed Won' || opp.stageName == 'Closed Won (R)'){
                oppIds.add(opp.Id);
            }
        }
        ContentDocumentLink cdl = null;
        List<ContentDocumentLink> cdl2 = [Select Id from ContentDocumentLink where LinkedEntityId IN :oppIds];
        if (cdl2.isEmpty()){
            opp.addError('Please ensure the contract is attached prior to closing this Opportunity');
        } else if(cdl2.size()<>trigger.new.size()){
            opp.addError('There is a mismatch in attachments and opportunities');
        }
    }
}

@IsTest
public class OpportunityHandlerTest {

    @IsTest public static void validateAttachment() {

        //Create Account and Opportunity
        List<RecordType> acctRecType = [SELECT Id FROM RecordType WHERE DeveloperName = 'Company' AND SobjectType = 'Account'];
        List<RecordType> oppRecType  = [SELECT Id FROM RecordType WHERE DeveloperName = 'AM_Channel' AND SobjectType = 'Opportunity'];
//        Test.startTest();  //why use "startTest" here?  we are only inserting test data
//        try{  //why are we doing a try in a test class for an object which is not related to the real code i want to test
            Account acct      = new Account();
            acct.Name         = 'Test Account1';
            acct.RecordTypeId = acctRecType.get(0).Id;
            insert acct;

            Opportunity opp     = new Opportunity();
            opp.Name        = 'test opp';
            opp.CloseDate   = System.today()+1;
            opp.StageName   = 'Initial Pitch/Demo';
            opp.Use_Case__c = 'Brand Tracker';
            opp.AccountId   = acct.Id;
            opp.RecordTypeId = oppRecType.get(0).Id;
            insert opp;

            ContentVersion contentVersion = new ContentVersion(
                    Title          = 'a picture',
                    PathOnClient   = 'Pic.jpg',
                    VersionData    = Blob.valueOf('Test Content'),
                    IsMajorVersion = true);
            insert contentVersion;

            List<ContentDocument> documents = [SELECT Id, Title, LatestPublishedVersionId FROM ContentDocument];

//create ContentDocumentLink  record
            ContentDocumentLink cdl = new ContentDocumentLink();
            cdl.LinkedEntityId = opp.Id;
            cdl.ContentDocumentId = documents[0].Id;
            cdl.ShareType = 'V';
            cdl.Visibility = 'AllUsers';
            insert cdl;
//if we are to do multiple tests, i would do all the test data above into a @TestSetup

            List<Opportunity> updateList = new List<Opportunity>();

            List<ContentDocumentLink> cdl2 = [SELECT Id FROM ContentDocumentLink WHERE LinkedEntityId =:opp.Id];
        //start the test when we are serious.
        Test.startTest();
            List<Opportunity> oldOpp = [SELECT Id FROM Opportunity WHERE Name = 'test opp'];
            for (Opportunity runUpdate : oldOpp){
                runUpdate.StageName = 'Closed Won (R)';
                updateList.add(runUpdate);
                System.debug(runUpdate.StageName);
                //update updateList;  //DML inside a for loop // NO
                System.assertEquals(documents.size(), 1);
                //this assert does nothing
                // except prove that we got a document when we queried the document object earlier
                // - it has no value in asserting our code except to say we got a document when using SOQL
            }
        //if we want a try catch, do it here whilst we are actually doing a DML for an update
        Boolean exceptionThrown = false;
        try {
            update updateList;
            //at this point i would also do a positive/negative test assert
        }
        catch(Exception e){
            System.debug(e.getMessage());
            exceptionThrown = true;

            // assert
            Boolean expectedExceptionThrown =  (e.getMessage().contains('Please ensure the contract is attached prior to closing this Opportunity')) ? true : false;
            System.AssertEquals(true, expectedExceptionThrown, e.getMessage());
        }
        Test.stopTest();
    }

}