Test Classmap Cleanup (for PHPUnit tests)

Tecla is a very simple PHPUnit test helper tool. It provides a workaround for unnecessarily registered classes of disabled/uninstalled extensions what happens if you are running PHPUnit tests (see #2006476: Strengthen tests by not autoregistering non-test namespaces of disabled modules.).

The workaround is pretty simple: it is driven by a service provider which removes the appropriate PSR4 mappings from Drupal's class loader. The solution uses reflectors, so it has some performance impact, but the benefit is a more realistic behavior during testing.