resourceloader: Create unit tests for ResourceLoaderContext

resourceloader: Create unit tests for ResourceLoaderContext

* Fix up one last use of global config vars in this class.
  Other places in this class already used $rl->getConfig().
  This way we don't inherit all of MediaWikiTestCase.

* Add unit tests covering all of ResourceLoaderContext
  except expandModuleNames and getImageObj (tested in better
  places already with the right @covers).

* Increase coverage for expandModuleNames(), add missing case
  of when modules are not in alphabetical order.

Change-Id: Id19b084d37a6c3a77b36e03509adffb6b156fee1