resourceloader: Use 'fallback' as default for ResourceLoaderTestCase

resourceloader: Use 'fallback' as default for ResourceLoaderTestCase

Follows-up fa05976f5f. This broke the Travis CI tests because
the Vector skin is not installed there (unlike in Wikimedia CI).

Make the test no longer reliant on that detail.

* Fix ResourceLoaderTestCase to use a default that actually
  works in plain core (previously 'vector' would be rejected
  as being an unknown skin, but then be re-normalised back to
  'vector' thanks to wgDefaultSkin. This was fixed in fa05976f5f,
  and the unknown skin now produces 'fallback' which is better.

* Update ResourceLoaderOOUIImageModuleTest to match this
  new default and also fix the test message while at it
  to mention 'skin', not 'image'.

Change-Id: I5fb69a6a38a42b1a5f325c0134e01ad880f65087