(Bug 51744) Add space around = chars in new headings

* Add new html2wt parser test that explicit tests this (although
  might be unnecessary given the various other tests that
  accidentally test this).

* To eliminate a lot of spurious failures, marked a bunch of
  wt-escaping tests for headings html2wt only.

* Updated wt-escaping for headings to test for new headings since
  the space around '=' chars prevents any '=' char in text from
  being interpreted as a heading char.
  Ex: <h1>=a=</h1> would serialize to "= =a= =" and hence the
  "=a=" does not need nowiki escapes anymore.

  Updated existing wt-escaping tests to differentiate between
  old/new headings.

* A bunch of failing tests are now off the blacklist because of
  marking those tests html2wt only.

Change-Id: Id81cad75e79df899b98d2f0d3d451196233b22a3
Merge "(Bug 51744) Add space around = chars in new headings"