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.
If Django was symlinked into site-packages the previous approach to discover
the tests subdirectory would fail. The revised version now always points to
the location of the source and not the import path.