mediawiki.jqueryMsg: Refactor "Match PHP parser" test suite

mediawiki.jqueryMsg: Refactor "Match PHP parser" test suite

Previously it was doing quite a few things wrong:

* Tests could potentially run in parallel. This was somewhat
  obscured by the asynchronous getMwLanguage() call being inside
  the synchronous each() loop. As such it basically first fired off
  the requests, and then handled them as they came back. They
  pretty much always come back in order (lucky) so the tests pass,
  but it was fragile.

* By default, jQuery.ajax() appends a cache buster ("&_{random}" query string)
  to requests with dataType 'script'. Disable this by setting cache=true.

Also:
* Store Promise objects instead of a low-level Callbacks list so that we don't
  have to wrap it. Instead, we simply chain from the $.ajax() promise, and use
  then() to feed the value instead of calling resolve() or fire().
* Use hasOwn() as extra sanity check (in case a random browser's non-standard
  Object.prototype methods match one of our non-standard language codes).
* Remove dead code "mw.messages.set( test.message )". This object has no message
  property (or anything else that it could be mistaken for). It just performs
  15 no-op in a loop. Follows-up 261aac63ec0777d6.

While the requests happen to (almost) always arrive in order, them running in
parallel put a lot of pressure on slow VMs using SQLite databases (which are
I/O bound). This causes a lock when queries from different processes overlap.

Change-Id: I0c5064780f80e1ddcc58ce32c390b2639b1f1ab6