Autodiscover extension unittests

Autodiscover extension unittests

Instead of requiring every extension that wants to add unit tests to
copy the exact same boilerplate over and over, let's just automatically
discover them. We now have an extension registry, so we know exactly
which extensions are loaded (this won't work for extensions not being
loaded through extension.json).

For each extension, we check to see if the directory "tests/phpunit/"
exists, and if it does, add those unit tests. If there is a
`UnitTestsList` subscriber already set, PHPUnit will automatically
de-duplicate test cases so we won't be running anything twice.

Change-Id: I6ec654ef2d8ee3630b121b1277b4ee21ba0b6cd4