resourceloader: Add tests for disallowing access to private modules

resourceloader: Add tests for disallowing access to private modules

* Add a test to confirm that the ResourceLoader::respond() logic
  works as intended.

* Remove the client code for preventing it from being loaded.
  This can never happen in production unless there is a bug.
  Instead of optimising to avoid a pointless request that only
  happens when the software is broken, instead optimise for when
  the software is not broken by just letting it happen. The server
  already handles it just fine.

  This was originally added in 2015 with 1dd73903726 to reduce
  logspam, but that was instead fixed in 6d6b037e122 by making the
  log message debug-only (because it's not a software problem,
  it's a client-error, e.g. a broken user script or a third
  party trying out different things on the load.php entry point).

  Removing this makes the client a bit smaller, too :)

Change-Id: Ic5420d9329a73514f4fc27baa46ae58d94addafb