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