Deprecate TestDiscovery test file scanning, use PHPUnit API instead

We are hitting a PHPUnit bug here, The file attribute of node of XML test list can be wrong. So we still need to read the file of a test class from the classloader.

I think we can still avoid the classloader. On it.

Very interesting side effect of this - since PHPUnit discovery actually executes the dataproviders, and includes in the discovery list as many test cases as the number of datasets provided for a test method, we can now be precise in the number of test cases run for each test class, and use that information in run-tests.sh instead of the current approximate count.

This looks really good and cleans up quite a lot of mess.

I went ahead and added @group #slow to mainly kernel tests that are now ending up towards the end of the run. This is not a fault in the ordering logic at all, it's just a side effect that some tests that happened to run earlier in a test run now could run later either purely because other tests moved around, or subtle changes in the method counting logic.

The debugging logic at the top of the test runs is great and I wonder if we should just keep that all the time.

I was thinking in a follow up we could introduce a “weight“ to #slow (i.e. #slow300, #slow200, etc) and further tune the start sequence.

@mondrake I think at the moment this is OK because we have less #slow tests than we run tests concurrently per-job (e.g. we might have 5 slow tests but start 8 at the same time), however it took a while for that to be the case and it might not be the case again, so definitely worth an issue. Even now it might save a few seconds which is worth having.

We can leverage on PHPUnit API also to determine the test description, instead of doing separate parsing. On it.

Done #14.

Filed 📌 Introduce a “weight“ to #slow tests Postponed re #12 and #13.

merged with HEAD after commit of 🐛 Extend PhpUnitTestDiscoveryTest to test also PHPUnit API Active

Thanks for review @mstrelan, added some of your suggestions and replied inline to some other ones; NW to address the remaining comments.