Testtools.TestCase eats DBBackup.save method

[Ed Cranford]

For some reason in our unittests, the save method belonging to trove.backup.models.DBBackup is overwritten to None. This happens inconsistently when running the tox tests, but consistently when the testr tests are run in the venv on their own.
I suspect it's a matter of timing or test ordering.

Asserting that the save method is not None at several points in the unit tests have shown that this change happens within the body of at testcase's setUp method

At present one workaround is to simply save the save method, and then restore it immediately in setUp. This is demonstrated in conductor's test_methods unittests.