Fix parserTests to serialize-to-html + reparse-to-DOM exactly once.

* The DOM was being serialized to HTML and parsed back multiple
  times on the wt2wt and selser paths which would change DOM
  semantics for HTML-pre (by stripping one newline each time).

* This patches fixes it to always do it exactly once.

* Fixes the HTML-pre test wt2wt failure.

* Speeds up parser tests run by over 20%.

Change-Id: Ic7e9a4a8fc92603cc96e25ea3ea9915f5abb37d8