Use RemexHtml as the tidy implementation for parser tests

Use RemexHtml as the tidy implementation for parser tests

* RemexHtml is the future of "tidy" in MediaWiki,
  so run our parser tests using it.

* This is a necessary step before we can make it
  the default in MediaWiki (T185753).

* Cleaned up a bunch of tests:
  (a) where html/php+tidy and html/parsoid match up,
      retained a html+tidy section and removed the others.
  (b) where html/php and html/php+tidy match up,
      retained the html/php section and removed the
      html/php+tidy section.

* Annotating tests with explanations where Parsoid & Remex
  output differ. This is usually because of two reasons:
  (a) Parsoid has Tidy-emulation code in some cases (which
      we can consider stripping away separately).
  (b) Parsoid does a bunch of cleanup on the DOM (which was
      probably done to emulate Tidy output, but which could
      probably be retained). Since Parsoid (in some form)
      will be default parser in the future, no reason to try
      to port this cleanup (in broken markup scenarios) into
      Remex.

* Left a bunch of FIXMEs for later followup.

Unrelated cleanup:
* Renamed a few tests since the functionality in Parsoid
  was fixed up. There is no more "implicit <td>" support.
  Those all now lead to fostered content.
* Fixed some clearly broken output in html/parsoid sections
  for some tests.

Co-Authored-by: Kunal Mehta <legoktm@member.fsf.org>
Co-Authored-by: Subramanya Sastry <ssastry@wikimedia.org>
Bug: T188167
Depends-On: I646dbabb3c2ed28c1ea72c5bd8f7f92d03f57c75
Change-Id: Ic7c34d57a300dbd36a37f03fbfe33391b2950b44