Fix bug introduced in NUnit2XmlOutput

Changes made in introducing new ResultStates broke NUnit2XmlOutput because the new states are not in the dictionary used to generate the NUnit 2 Xml results.

Rather than adding the new states to the dictionary, I introduced code that does the translation using switch statements. This allows a default to be used, which will be more robust in the face of new user-created ResultStates.

It's unfortunate that we are not testing NUnit2XmlOutput other than to validate the XML. An older test is commented out because it no longer combines. This needs to be re-introduced but at the moment the critical thing is to fix the bug that was just introduced.