[SPARK-4631] unit test for MQTT

Please review the unit test for MQTT
Who is running the broker? Also this port is hardcoded. There is a small, non-trivial chance that this port may not be free (in Jenkins, where multiple series of test maybe running in parallel) causing the server to not bind thus failing test. Can you find a free port (see [FlumeStreamSuite](https://github.com/apache/spark/blob/master/external/flume/src/test/scala/org/apache/spark/streaming/flume/FlumeStreamSuite.scala#L78)) and use that instead?