`wptexturize()` adjustments:
* Only place an apostrophe before a number when it has exactly two digits.
* Never match '99' with the single prime pattern.
* Always assume '99' is an abbreviated year at the end of a quotation.
* Add unit tests.
* Resolves the unit test broken in [28721] for #8775.

See #26850.


git-svn-id: https://develop.svn.wordpress.org/trunk@28761 602fd350-edb4-49c9-b593-d223f7449a82