[Policy] Frequent re-testing of RTBC/CNR patches for whether they apply and/or linting

#1952058: [META] Retesting stale RTBC core patches got deployed recently which is fantastic.

There's a couple of other things that would help to prune the core queues further.

The test on git apply + php -l is more or less instant compared to running the full test suite. Some 24 hour periods see more than 50 core commits, so if we were able to re-test just for those conditions, it would catch patches very early. What it would also enable is re-testing the CNR queue, which is considerably longer (but could get significantly shorter, at least I hope!).

If we also did #2189317: Get a PHPUnit job type running that is separate and independent from the Simpletest Job type, fix the d.o integration so that we can run the PHPUnit tests independently, it might be possible to add PHPUnit to the re-tests as well - whether that's a

Additionally, if we added PHPUnit as a separate step per those tests take around 50 seconds, so those could also happen in a frequent test bot without a lot of additional infrastructure load. That might lull people into a false sense of security though, not sure.