HTML: COOP+COEP blob URL popup

[annevk]

Part of #18354.

[foolip]

wpt-firefox-nightly-stability is failing on Taskcluster

[zcorpan]

I'm having a bit of difficulty assessing the correctness of the test without a spec or an implementation. :)

I didn't find anything clearly wrong though, if that helps.

[annevk]

FWIW, this tries to open a same-origin blob: URL from a COOP+COEP environment and expects that to not replace the BCG.

If we don't want to break blob: URLs this is the simplest case that would have to work for popups, though workers are probably a more relevant test come to think of it.