@Override
        public void submitTask(Runnable task) {
            checkTaskAcceptance();
            tasks.add(new TaskDescriptor(task, 0, ++sequenceCounter));
        }
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