Extensions may add tests by directory

The UnitTestsList hook can now be used to add entire directories of
tests, à la phpunit.xml's <directory> tag.  The test suite is built by
recursively scanning the directory for any files ending in "Test.php".

TODO:
* Update online hook documentation.
* Generate and autoload a classmap for scanned directories.

Bug: 70630
Change-Id: I3089372f9d7c645e16ff0984a959f982a3bc639f
Merge "Extensions may add tests by directory" into REL1_24