Test extensions with vendor and proper mw branch

This switch most MediaWiki extensions unit test to use the Zuul cloner.
It has some side effects:

- extensions now have vendor repository fetched
- job will attempt to fetch the branch of the proposed change. Which
  mean submitting a patch to an extension REL branch will run tests
  using mediawiki/core with that release branch (previously it was
  always the master branch).
- have mediawiki-core-lint shared by mediawiki/core and all extensions.
  This way the jobs share the same gating queue.

Change-Id: I6f881d6db3f1752823a0d6b97b41d561e58722a5