Placeholder test writing for WHATWG relative reference proposal

[cookiecrook]

There's a WHATWG issue proposing allowing relative references rather than limiting them to IDREF or IDREFS.

• Improve DX of element reference attributes by allowing relative references instead of only ids whatwg/html#10143

It seems possible that the existing agreement on Frozen Element Array reference may end up being sufficient, but if not, I'm filing this WPT task as a placeholder for future WPT test writing...

No action required until the linked issue makes progress.