How to see if the test works?
See https://www.drupal.org/node/2116263.

You either need to run it inside the core folder, or (easier IMHO), you need to pass the -c core argument, so it finds the phpunit documentation.

Also note that only a part of Drupal's tests are phpunit, others are based on our own test framework and you need to run them with run-tests.sh or through the UI.

That said, you only need to run core tests if you work patches for Drupal core. Drupal's tests always path, nothing is added unless it is passing all tests.