TEST_UTIL_System_Omit.cls

@isTest(SeeAllData = false)
private class TEST_UTIL_System_Omit
{    
    static testMethod void testOmit()
    {
        
        List<Account> accounts = new List<Account>
        {
            DAL_Account.insertItem(),
            DAL_Account.insertItem(),
            DAL_Account.insertItem()
        };

        Account a1 = DAL_Account.insertItem();
        Account a2 = DAL_Account.insertItem();

        accounts.add(a1);
        accounts.add(a2);

        Set<ID> idList = UTIL_System.extractListIds(accounts);
        Set<ID> idToBeOmited = new Set<ID>{a1.ID, a2.ID};
       
       Test.startTest();
       Set<ID> results = UTIL_System.omit(idList, idToBeOmited);
       List<Account> omitWhereResutls = UTIL_System.omitWhere(accounts, DAL_BaseObject.ID_FIELD, (Id)a1.get(DAL_BaseObject.ID_FIELD));
       Set<ID> concatResults = UTIL_System.concat(results, idToBeOmited);
       Test.stopTest();

       System.assertEquals(3, results.size(), 'Should be left 3 Ids in list');
       System.assertEquals(4, omitWhereResutls.size(), 'Should remove one account from list and be left 4 items in list');
       System.assertEquals(5, concatResults.size(), 'Should add 2 into set and be size of 5');
       
    } 
}