In `wptexturize()`, ensure that texturization does not corrupt contents of HTML elements, HTML comments, and smartcode attributes.

Adds a variety of unit tests/assertions.

Props miqrogroove.
Fixes #12690, #8912, #27602.


git-svn-id: https://develop.svn.wordpress.org/trunk@28727 602fd350-edb4-49c9-b593-d223f7449a82