Simplify L3 HA unit test structure

Previously we had two fake test plugins: With and without agents.
I removed the one without agents and merged two test cases.
This allows test writers to add additional tests more easily
without mocking things that should not be mocked.

Change-Id: I6f899f86034145ac88b5c2e0f3036f7477f9702f
Merge "Simplify L3 HA unit test structure"