Phabricator: run unit test

I want to write a custom Unit Test for Phabricator.

For that purpose, I checked out the documentation for Writing Unit Tests in Phabricator and created a file at ./phabricator/src/infrastructure/testing/testcase/PhabricatorTrivialTestCase.php (for using the trivial phabricator test case) with the following content

class PhabricatorTrivialTestCase extends PhabricatorTestCase {

  private $two;

  public function willRunOneTest($test_name) {
    // You can execute setup steps which will run before each test in this
    // method.
    $this->two = 2;
  }

  public function testAllIsRightWithTheWorld() {
    $this->assertEqual(4, $this->two + $this->two, '2 + 2 = 4');
  }

}

When I try to run it, I get the following message.

$ cd ./phabricator
$ arc unit src/infrastructure/testing/testcase/
No tests to run.

Why can't I run the test as documented? Is there some step missing?