TestCollection.m: Disable tests that use -[COEditingContext libraryGroup] and related methods, as those methods have broken memory management (see comment in -libraryGroup in COLibrary.m), and they fail as expected on GNUstep (though spuriously work on Cocoa).

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10477 7b3f36aa-e6db-49c1-8b4c-66eac1997e64