Adds addClassCleanup classmethod to BaseTestFixture.

* addClassCleanup is a version of addCleanup that can be used from
  within classmethods like setUpClass that will run it's cleanup
  tasks after tearDownClass, or if setUpClass fails (if it was used
  inside setUpClass).
* Cleanup tasks are run in reverse order, like addCleanup.

Change-Id: If0007387d2711bab9a4d63bd9fc5b3242fb914e2
Merge "Adds addClassCleanup classmethod to BaseTestFixture."