Ticket #23778 -- Add a code snippet in the advanced testing documentatio...

...n showing how to run tests outside de test runner.
I think we should be consistent in assuming that the Python path hierarchy starts at the top level, meaning this path should be `'tests.test_settings'` instead. In fact, because the directory of the executed script goes on the path, we have overlapping paths here and either will work.

I think it might be better to move `runtests.py` to the top level, which gets rid of the overlapping paths (and also incidentally removes the requirement to use `pip install -e` or set `PYTHONPATH`). I know this isn't quite as consistent with how Django does it, but frankly the Django approach has caused problems with the overlapping paths in the past, so it might not be the best model to follow.