Fixed #13332 -- Corrected the cleanup code in the test client to avoid a...

Fixed #13332 -- Corrected the cleanup code in the test client to avoid a refcounting problem with signal handlers. This is a fix for the benefit of PyPy's hybrid GC. Thanks to Alex Gaynor for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12964 bcc190cf-cafb-0310-a4f2-bffc1f526a37