Retain accumulated separator text across node boundaries

* WTS was emitting accumulated separator at node boundaries
  (after serializing children).  However, this is not necessary
  and can sometimes lose separator text when a node's children
  is only made up of separator text.

  This code might have been required at some point early in the
  WTS refactoring from a token-based strategy to a DOM-based
  strategy.  But, with a lot of bug fixes, fixing of separator
  constraints, this no longer seems to be necessary.

* This patch eliminates the regressions introduced by 2e8619ad
  (noted in the commit summary there).  Updated blacklist
  accordingly.

  This fix also eliminates the regressions found in RT testing.
  Check URL below:

http://parsoid.wmflabs.org:8001/regressions/between/33cbf927efcd2fc502524a6c2e114886cbd46eee/2e8619ad498033b1504e4e2e17fe6c503305ba18

* In addition, with this fix, 'migrateTrailingNLs' DOM pass in
  the front-end may not be necessary either unless VE code
  depends on it.  'migrateTrailingNLs' was added to deal with the
  sensitivity of the serializer as to where newlines showed up.
  Once again, testing shows that there is just one wt2wt regression
  (which might be because of something else).  To be investigated
  separately -- but recording it here for now.

Change-Id: I025c749f92a09fc11eeb9a574e580f5ceb486602