[1.2.X] Fixed #14148 -- Ensure the admin_script tests always use an absolute...

[1.2.X] Fixed #14148 -- Ensure the admin_script tests always use an absolute path; PyPy (and sometimes CPython) return relative paths for __file__. Thanks to Alex Gaynor for the report and patch.

Backport of r13618 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13619 bcc190cf-cafb-0310-a4f2-bffc1f526a37