(Bug 56646) Edge-case bug fix in migrateTrailingNLs dom pass

* migrateTrailingNLs had a bunch of tests for when to migrate
  trailing newlines across meta tags. But, it never really
  tested for zero-width tsr. This cause a newline to get migrated
  across a <meta placeholder> tag that gets inserted for missing
  images (which is a bug and introduced an off-by-1 error in DSR
  computation of the parent node). This bug showed up in our local
  testing because the images referenced by our local copies of
  the BO page are all non-existent locally.

  So, in production, this bug would have only surface when a missing
  image happened to end a paragraph and had newlines preceding it
  and so, really an edge case.

* That said, the bunch of special-case tests should not be necessary
  anymore -- they were probably present in earlier versions of the
  code and are probably fragile.

  No change in parser tests -- should I concoct a test for this?

* But, since is a more radical change, this should be put through
  the paces via roundtrip testing.

Change-Id: I92710ce547906c0fc7eb531aebb8b5d3434657ce