Attributed string tests: refactor to avoid directly accessing COAttributedStringAttribute.htmlCode property

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10285 7b3f36aa-e6db-49c1-8b4c-66eac1997e64
Undo: remove unused method -rewrittenCommandAfterCommitInContext

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10286 7b3f36aa-e6db-49c1-8b4c-66eac1997e64