Fix separator stripping bug in serializeTextNode in WTS.

* The regexp that stripped newlines was different from the one
  that stashed it in handleSeparatorText which led to extra newlines
  in some tests.

* Updated the "HTML-pre test with embedded newlines" test to
  expose the bug before the patch. Added a html/parsoid section.

  This test now passes html2wt and html2html modes.

  This test fails in wt2wt mode which is likely a bug in the
  parserTests script since when the wt2wt test is run in isolation,
  the output is correct.

Change-Id: Iab20ba1e8fc19d10fdc046452f4d625ada56ec56