Create kafka-junit test for KafkaAnomalyDetectorMapper

[williewheeler]

Currently the unit test is based on Kafka's test-utils package, but kafka-junit provides deeper support as it allows us to set up a mock broker with input/output topics that we can use. Update the existing unit test to use kafka-junit.

[williewheeler]

May not want to do this. See comment on issue #369.