Use debug level logging in unit tests, but don't save them.

We want to try formatting debug level logs so we can gate on any
formatting errors, but we don't want to save the debug level logs as
there are just too many of them. So use fixtures.FakeLogger running at
INFO level and a custom NUllHandler at DEBUG level.

Not Using fixtures level parameter since that sets the log level for the
entire logger and not just the individual handlers.

Don't inherit from logging.NullHandler since it isn't in python2.6

Change-Id: I7317092c0a2bad8efd11ece3ba521d58c2acae42
(cherry picked from commit 158ac3d6c6ed29b3c3bed1448993d5276b0fe366)