LPS-44135 Mail listener should always ben used in a syncronious enviroment, so we should explicitly show that to developers. We should extend and reuse the SynchronousDestinationExecutionTestListener to avoid import 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