[1.2.X] Fixed #14508 - test suite silences warnings.

Utility functions get_warnings_state and save_warnings_state have been added
to django.test.utils, and methods to django.test.TestCase for convenience.

The implementation is based on the catch_warnings context manager from
Python 2.6.

Backport of [14526] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14527 bcc190cf-cafb-0310-a4f2-bffc1f526a37