Replace PersistenceMixin with PersistenceHelper.
Rename FooTestCase to TestFoo for things that are not base classes, various related cleanups.
Resolve a bunch of TODOs the previous commits added.
Rename a test that was missed previously.
Replace uses of get_stubbed_worker() with WorkerHelper.
Move fake_amq_message() to the only test module that uses it.
Get import_skip() from helpers, not utils.
Add some deprecation warnings to old test utils.
Fix dispatcher test persistence handling.
Merge branch 'develop' into feature/issue-647-persistence-test-helper
No more passing test classes to helpers.
Fix broken test.
Tiny cleanups.
Pass use_riak param into helpers the create persistence helpers.
Merge branch 'develop' into feature/issue-647-persistence-test-helper
Merge branch 'feature/issue-647-persistence-test-helper' into develop