Tweak mockAPI to yield image paths consistent with PHP parserTests.

This is one step in an effort to make the PHP and Parsoid parserTests
correspond more closely.

Change-Id: I32dd818e1a94e3b187608234fbf3203a52921f99
Reorganize image-related parser tests.

Tag test with php/parsoid and move the parsoid versions of the image
tests directly adjacent to the php versions, so we can easily see that
the two versions are testing the same features.

The image-with-external-link tests fail wt2html, due to bug 49293.

Change-Id: I49e7d387c41320ba7e4157a3e6e84686ae13c370
Merge changes I49e7d387,I32dd818e

* changes:
  Reorganize image-related parser tests.
  Tweak mockAPI to yield image paths consistent with PHP parserTests.