Fix excess nowiki escaping in list elements

* "<ul><li> 

: a</li></ul>" was getting serialized to
  "* 

<nowiki>: a</nowiki>" because of a buggy wt-escape
  handler for list elements.

* This patch fixes it and updates wt_escape.tests.txt
  TODO: Migrate recent changes to this file to parserTests.txt

Change-Id: I495873740a8ad0cd4ce471cd57c514378e7d88a5