Fixed #8244: Modified the temporary directory used by file storage tests so...

Fixed #8244: Modified the temporary directory used by file storage tests so that mutliple test runs can be performed in parallel without conflict. 

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8376 bcc190cf-cafb-0310-a4f2-bffc1f526a37