Migrate trailing newlines across zero-width-wt DOM nodes

* Trailing newlines can be migrated across DOM nodes that
  serialize to zero-width wikitext. For example, in the
  wikitext below, the implicit <td> inserted in the tokenizer
  after the "|-" <tr> has zero width wikitext.
{|
|-
{{echo|<tr><td>foo</td></tr>}}
|}

* The above test is now part of parser tests (in a core commit
  awaiting review and merge) and passes.

* This fix also eliminates semantic RT errors from en:The better man

Change-Id: I7890ac9eb11e774cd7bf01d229c1058904585ada