enable test.php webrunner again and fix a missing namespace

A few months back the test.php testrunner file got removed during a 2.4 merge. Probably due to convenience during a merge conflict.
Anyway, during a discussion some including me saw the usefulness in 3.0 still supporting the webrunner as it eases the start into testing.

This moves the test.php webrunner back into 3.0.
For it to work I had to first add a missing use, though.

Note that this might not be the best approach regarding the current overhead in TestSuiteDispatcher::run() - also in light of its possible removal. Open for discussion on how to best proceed.