Allow setting default thumb size in parser tests

The mediawiki default thumb size is 180px.  The Parsoid default thumb
size is 220px, to match the default thumb size for most WMF wikipedias
(see https://bugzilla.wikimedia.org/show_bug.cgi?id=43336).

This discrepancy leads to inconsistent image-related test output.
Allow a test to set an explicit default thumb size with the
'thumbsize' option so that it is possible to write consistent tests.

Change-Id: Ib764d1f1660a50caaf8f0ff245822d1d1a1d264e
Merge "Allow setting default thumb size in parser tests"