Publish API callout mock, so customer org tests will work.

[waltjones]

Salesforce won't let users of managed packages call Test.setMock for callouts in the managed package. Yet they also require that all callouts are mocked.

This PR provides a wrapper so Test.setMock can be called within the package, making it possible to mock the API calls.

The Rollbar package tests are also updated to use this helper class internally.

The user tests can use:

rollbar.RollbarTestHelper.setDefaultMock();

Or

rollbar.RollbarTestHelper.setMock(new CustomMock);

Example:

@isTest
public class CalloutTest {
    @isTest
    public static void calloutIsMocked(){
        // Set the test mock.
        rollbar.RollbarTestHelper.setDefaultMock();

        // Also initialize a test token. (Can be any string.)
        insert new rollbar__RollbarSettings__c(rollbar__AccessToken__c = 'test-token');
        
        Test.startTest();
        // Any Rollbar callouts (e.g. rollbar.Rollbar.log()) should work.
        Test.stopTest();
    }
}

[ajtran]

Looks alright to me!