mediawiki.Title: Generalise filename-matching in newFromImg

Both core and extensions can (and do) modify the thumbnail string
e.g. to add page numbers, languages, etc.

As bawolff notes in the commentary to bug 55963, the assumption
that the filename itself appears twice is likely to be violated
from time to time, but for the moment it suffices to fix the
apparent regression described in that bug report.

Also includes some additional test cases.

Bug: 55963
Change-Id: I9bb29a24f9bf50924699b913e31f44689906c525
Merge "mediawiki.Title: Generalise filename-matching in newFromImg"