Add all images to MockAPI for parser tests; fix up some image tests.

Clarify the SVG tests which used 'width=200' as a *caption*.  That's
confusing.  'width' isn't a valid image option.  Use 'caption'
instead.  The SVG tests now fail because we don't (yet) support the
'lang' option -- previously the wt2wt tests were passing because we
were treating the text as a placeholder. Fixed in
Iae50f6e4948844b94a66f8437e12e05aa3ec1685.

Add Foobar.svg and Thumb.png to the MockAPI used to get image attributes.

Tweak some parsoid-specific image tests to make them round-trip cleanly.

Change-Id: I7bc80b6280de17eab807955be18cca93fab695fc