[1.2.X] Fixed #14254 - More tests for storage backends

Thanks to steph for the patch, also to tobias for cleanups.

Backport of [13852] from trunk

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13854 bcc190cf-cafb-0310-a4f2-bffc1f526a37