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

...n showing how to run tests outside de test runner.
In fact, again in the interests of keeping things simple, I'd suggest just hardcoding here instead, so it's more clear what's going on: `runner.run_tests(['myapp'])`. It's better to show the simple version so people don't cargo-cult more complex stuff they don't really need, thinking it's required.