TestAttributedStringWrapper: refactor in to a generic NSTextStorage test, and run it against 3 different implementations: COAttributedStringWrapper, a trivial NSTextStorage subclass that uses NSMutableAttributedString as a backing, and the private text storage used by a NSTextView instance.

git-svn-id: svn://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@9975 7b3f36aa-e6db-49c1-8b4c-66eac1997e64