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

Please see [#23742](https://code.djangoproject.com/ticket/23742).
For the sake of thoroughness, could you update this test to verify it does the right thing for each test class type: TestCase, SimpleTestCase, and unittest.TestCase?

Also, I just noticed ``test_discovery_sample2`` app is unused. It looks like we added it on accident. Instead of using ``test_discovery_sample``, you could add the test classes there. They would be isolated and I think the benefit is that it would be easier to reason from the test that reverse really does the right thing.

I know `test_testcase_ordering` reuses the `test_discovery_sample` app, but I think that approach makes things muddy, especially since it means modifying unrelated test methods.

Also, good job on this patch. I think it will be a nice addition.