LPS-44135 Create a new listener to prepare test to use the fake mail server (dumbster)
LPS-44135 Mail listener should always been used in a synchronous enviroment, so we should explicitly show that to developers. We should extend and reuse the SynchronousDestinationExecutionTestListener to avoid importing too many Listeners
LPS-44135 Use the new listener in MailEngineTest
LPS-44135 Use the new listener in Subscription tests
LPS-44135 Use the new listener in User Notification tests
LPS-44135 Delete unused Base class
LPS-44135 SF