qunit: Enable PHP debug, fix default config, dump load.php

* Make PHP log and show all errors.
* Reset wgScript, wgStylePath and wgLogo so that they don't
  still use the default wgScriptPath but are re-computed by
  Setup.php
* Dump load.php as well for errors not visible on index.php

Change-Id: I8daf47d82095423346e8bd3cfe8a1f66affcf6f2