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

...n showing how to run tests outside de test runner.
Let's keep things simple by only referencing  pip, not setuptools; in general pip is the Django standard. I think we should show `pip install -e .` as the demonstrated example; we can mention `PYTHONPATH` as an option too, but `pip` should be mentioned first and shown as the example.