[Zotero_TranslatorTester] Move _generateDiff  and _compare out of prototype

Just a nit, I guess. I was using _generateDiff from Scaffold, which I could have done by `Zotero_TranslatorTester.prototype._generateDiff`, but that looked a bit hackish. Since neither _generateDiff nor _compare depend on Zotero_TranslatorTester object itself, I figured they would actually fit right in with the other non-prototype methods.

The `keepValidFields` option on _sanitize is also for something in Scaffold. I'll submit the Scaffold changes in a bit.