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.