Fixed #23742 -- Added an option to reverse tests order.

Please see [#23742](https://code.djangoproject.com/ticket/23742).
Thanks, made it available with ``test`` and added some docs. Here's a diff with [the changes](https://github.com/wrwrwr/django/commit/f946814e31c2411aeb0c5c31a606c24f668ed342).