Include name/level in unit test log messages

Use a more informative format than just the raw message string in log
messages captured by fixtures.FakeLogger during unit testing.  For
example, this changes cryptic unit test output such as

    pythonlogging:'': {{{
    214 -> 215...
    done
    215 -> 216...
    done
    }}}

into:

    pythonlogging:'': {{{
    INFO [migrate.versioning.api] 214 -> 215...
    INFO [migrate.versioning.api] done
    INFO [migrate.versioning.api] 215 -> 216...
    INFO [migrate.versioning.api] done
    }}}

which instantly makes it clearer that these numbers come from database
migrations.

Change-Id: I56b6fa03892e6d865ccb036fdf75d2bb499b6c93
Merge "Include name/level in unit test log messages"