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

Please see [#23742](https://code.djangoproject.com/ticket/23742).
Surely, modifying those existing cases wasn't making them any better.

Reset the ``test_discovery_sample`` and changes in tests using it, added a "naturally" ordered collection of cases to ``test_discovery_sample2`` and strengthened ``test_reverse`` with some more asserts. Just the [latest changes](https://github.com/wrwrwr/django/commit/ef495319abdb1bcfad0f1f55630300723c3df27b).