Storm-kafka tests leak resources due to retained references to KafkaServer

Since JUnit keeps a reference to each test class until all tests for a module have run, the KafkaTestBroker fields are not eligible for garbage collection until all tests have run. KafkaTestBroker keeps a reference to KafkaServerStartable/KafkaServer, which allocates resources that are also not eligible for collection. This very quickly causes surefire to run out of memory if this class is used more than a few places.