1. Migrates topology builder mock in TaskManagerTest to mockito.
2. Replaces the unit test to verify if subscribed partitions are added
to topology metadata.
3. Modifies signatures of methods for adding subscribed partitions to
topology metadata to use sets instead of lists. This makes the
intent of the methods clearer and makes the tests more portable.
Reviewers: Christo Lolov <lolovc@amazon.com>, Matthias J. Sax <mjsax@apache.org>
