[1.2.X] A few test optimizations; using native unittest where no...

[1.2.X] A few test optimizations; using native unittest where no Django-specific TestCase features are required.

Backport of r13935 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13943 bcc190cf-cafb-0310-a4f2-bffc1f526a37