[1.1.X] Fixed #12123 -- Refactored dateformat tests to use unittest. This is...

[1.1.X] Fixed #12123 -- Refactored dateformat tests to use unittest. This is needed so that we have a reliable teardown that can reset the timezone at the end of the test.

Backport of r11693 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@11694 bcc190cf-cafb-0310-a4f2-bffc1f526a37