A bunch of the unit tests in the hydrator-plugins repo use plugins from the core-plugins module in their unit tests. This is bad because it means the unit test for cassandra is really not just testing cassandra, it is also testing another plugin. And if that plugin changes, it may break the unit test.
Instead, the mocks used by the cdap apps should be pulled out into a tests module that can be used by the app unit tests as well as by plugins unit tests.
Added a hydrator-test module that contains mock plugins for unit testing other hydrator plugins.
This change will help with CDAP-4656, by allowing removal of StreamBatchSource (and ETLUtils) code from the CDAP repo.