@isTest
private class CalculateShippingDateTest {

@testSetup static void setupData(){
        Case caseOne = new Case(Status='New',
                                Origin='Web',
                                Subject='BusinessHoursTest');
        List<Case> cases = new List<Case>();
        cases.add(caseOne);
        insert cases;
    }

    @isTest static void addGMT(){
        // retrieve our test case from the database
        List<Case> cases = [SELECT Id, Subject, Shipping_Date__c
                            FROM Case
                            WHERE Subject='BusinessHoursTest' LIMIT 1];
        // Call the addTwoBusinessDayToCase to calculate and populate the shipping date on our case
        CalculateShippingDate.addTwoBusinessDaysToCase(cases);
        // Calculate our expected Shipping Date for comparison
        DateTime expectedShippingDate = BusinessDays.addGmt(System.today(), 2);
        Date shipDate = expectedShippingDate.Date(); // we only care about the Date, not the Date + Time for our use case
        Test.startTest();
        // retrieve the updated record
        Case fetchedCase = [SELECT ID, Shipping_Date__c
                                   FROM Case
                                   WHERE ID IN :cases];
        // run the test - compare the expectedShippingDate to the actual Shipping_Date__c calculated on our cases
        System.assertEquals(shipDate, fetchedCase.Shipping_Date__c, 'Expected 2-business days to be added.');
        Test.stopTest();
    }
}