DOMParser: Tweak tests for interface of returned document
See https://github.com/whatwg/html/issues/11372. This matches all browsers.
Yeah, as-is this would require specification changes. Which we should probably make, but figuring out how to solve this was somewhat non-trivial I believe.
@domenic @annevk Ok I just removed those tests for now. Those tests would also not be correct if we removed the XMLDocument interface (because they would do instanceof undefined).
(instanceof Document is tested in checkMetadata)
Per https://dom.spec.whatwg.org/#xmldocument we do define that XMLDocument exists. So we should go with the original approach and update the specification to match.
Alright, done!