(Bug 57469) Fix for nowiki escaping of ext-tag like text

* Extension tags whose opening and closing tags were spread
  across different DOM nodes (as opposed to being in the same
  text node) were not being escaped. This patch fixes it.

* Added parser test to capture this requirement.

Change-Id: I2196be2e87f35829b8d31a6f37f61c6937ee0add
Merge "(Bug 57469) Fix for nowiki escaping of ext-tag like text"