Diff/COArrayDiff.m: Start adding test cases in Tests/Diff/TestArrayDiff.m, fix a bug

git-svn-id: svn://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@9813 7b3f36aa-e6db-49c1-8b4c-66eac1997e64