Adapt the <gallery> parser test for $wgResponsiveImages

$wgResponsiveImages was added and enabled by default in 966cda.
Makes the wiki to use thumb urls in some cases (up to 2x)
where we showed the original before.

Change-Id: I7b399c7d8ef9e47f8cd5c999738ec349e1585777
Merge "Adapt the <gallery> parser test for $wgResponsiveImages"