Adding phpunit as a composer dependency makes it a lot easier for running tests as you don't need to have it working in your path and you can be sure that every time tests are run it's the same version of phpunit (4.0 is now out and PHPUnit's release cycle is a lot faster paced than it used to be).

http://phpunit.de/manual/current/en/installation.html#installation.composer but add https://github.com/phpbb/phpbb-website/blob/master/composer.json#L58-L60 also so that the phpunit exectuable gets put in bin/ in the project.