Added a skip for a test that fails in Oracle. Unlike other backends, Oracle...

Added a skip for a test that fails in Oracle.  Unlike other backends, Oracle does not allow duplicate rows where there is a unique_together constraint for which some but not all of the columns are NULL.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15777 bcc190cf-cafb-0310-a4f2-bffc1f526a37