Added executor framework to sync prod and tests

+ Use a TestableExecutor in the production code to allow tests to
sync up and block so the state of the system can be tested. In the
tests the executor can wait until the production code hits a
milestone to ensure that the system is in a proper state for testing.

+ The current implementation only attempts to synchronize between one
production thread and one test thread. Extend the TestableExecutor
interface to perform additional synchronization.

Change-Id: Ie6fc64392e402330ab66c6f2cd0ec22200ebbdea