public void expectAndProcessTasks(int expectedTaskCount, long[] taskRunAtTime) {
            expectAndProcessTasks(expectedTaskCount, expectedTaskCount, taskRunAtTime);
        }
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