Improvement to serialization of text nodes in selser mode.

* Removed a hard check of 1 child (text node) of a block node
  and replaced with a more robust 'all-children-are-text-nodes'
  test that also works with selser that introduces diff-marker
  meta nodes.

* 19 additional selser tests now pass.

Change-Id: Iefcb0caab24104231203503dc1a628dadd6bd633