private MessageBusVisitorSession createVisitorSession(MockSender sender,
                                                          MockReceiver receiver,
                                                          MockAsyncTaskExecutor executor,
                                                          VisitorParameters visitorParameters)
    {
        return createVisitorSession(sender, receiver, executor, visitorParameters, null, new MockClock());
    }
Mock the executor to keep things nicely single threaded for the testing.
No need to synchronize things here since we don't use multiple threads.
documentapi/src/test/java/com/yahoo/documentapi/messagebus/MessageBusVisitorSessionTestCase.java