Correctly handle adjacent text and p-node children of block nodes

* This patch cleans up some FIXME and unhandled cases around
  serializing text-node siblings of p-nodes when they happen to
  be children of block-nodes. These special cases come about
  because of p-wrapping special cases for blocknodes in the parser.

* This patch also fixes bug 51087 -- new html2wt tests added for
  this which pass.

* 2 more wt2wt tests now pass (alongwith associated selser tests).

* 1 selser test now fails. The lost newline is semantically insignificant
  and is not something to be concerned about in this example. It is
  heavily edited HTML that originally had lot of tag nesting correction.

Change-Id: I5ede7a9bbd1e976b8cde3f63b4f688e9f148a059
Merge "Correctly handle adjacent text and p-node children of block nodes"