cater for running tests from another directory than tests - applying existing pattern to newly added test_testcase_ordering as well

Thanks aptiko for the reporti and Tim Graham for the review.

This was used for python 2.6 and unittest2 compatibility. It was
forgotten when the code that used it was removed.

Added test to verify an empty label performs discovery on the current
working directory.

Thanks thepapermen for the report and Carl Meyer for the review.

Added DiscoverRunner.test_suite and .test_runner attributes.

Thanks tomchristie for the suggestion and jcd for the patch.

Silenced warnings caused by the deprecation of django.utils.unittest.

Thanks Preston Timmons and Carl Meyer for their advice.

Fixed #20680.

Refs #20680.

Thanks to Preston Timmons for the bulk of the work on the patch, especially
updating Django's own test suite to comply with the requirements of the new
runner. Thanks also to Jannis Leidel and Mahdi Yusuf for earlier work on the
patch and the discovery runner.

Refs #11077, #17032, and #18670.