Make sure to always fix up the MEDIA_ROOT after admin tests.

If UpdateTests.testManualUpdatesRequiredBadUpload fails (say, due to something
being rendering the base.html template), it will leave settings.MEDIA_ROOT and
siteconfig's site_media_root in a broken state. This breaks a bunch of other
tests, which is highly confusing.

Testing done:
Ran into this case when working on my django1.5 branch, and this fixed it.

Reviewed at https://reviews.reviewboard.org/r/4584/