(Bug 63642): Cleanup treebuilder fixup of formatting elts.

* First pass refixing treebuilder-fixed DOM where formatting elts
  (<small>, <b>, etc.) wrap <figures> inside tables. This patch
  moves the formatting elt inside the caption.

* Added a test snippet that is tested in wt2html, wt2wt, and
  selser modes. wt2wt will always fail, but will enable selser
  tests (which if passing will get blacklisted since results
  will differ from wt2wt) and will let us catch regressions in
  those results.

Change-Id: Idaaf46565ad277289b84a4711da38ee79f2d483f
Merge "(Bug 63642): Cleanup treebuilder fixup of formatting elts."