Ext-tag text nested in ext-tag of the same name dont need nowiki escapes

* Nested extension tags are not supported in the front-end (ref-tags
  inside templates are an exception but we dont serialize template
  content directly and hence that use case is not relevant here).
  So, there is no need to wrap ext-tag text in extension tag nodes
  of the same name.

* This was not supported till now since this is probably not a big
  deal in terms of frequency in real world pages.  But, given the
  sensitivity to nowikis, fixing this use case since it is easy to fix.

* Fixes a bunch of tests around serializing nested ref tags.
  Updated blacklist

Change-Id: I4c42c783a9d973ba43a47fc5526097d1cd0ff082