mediawiki-vendor-integration -> mediawiki-phpunit

mediawiki-vendor-integration was a lame name, lets rename it
mediawiki-phpunit which is straightforward.

Once merged:
* we will want to delete the old workspace from all slaves to
  reclaim disk space.
* the first run on a slave will takes up to 3 minutes to clone the
  mediawiki/core.git repository

Bug: 72787
Change-Id: I127b352b951b9f67efe0f098968a9e59faaa78b7