Add test cases for null registry element upon adoption

[ja-y-son]

Add test cases that covers the scenarios in whatwg/dom#1437

[ja-y-son]

@annevk PTAL, thanks!

[annevk]

I think we should have some tests where you just call adoptNode(). And some tests with attachShadow() instead of declarative shadow roots.

And some tests where we initialize() various parent nodes in this null/global shadow tree with a scoped registry prior to adoption.

[ja-y-son]

Added more test cases to hopefully cover most of the adoption cases that we're interested in this spec change. I haven't implemented this part of spec change in chromium so I tried my best to verify that each test case is asserting the correct expectation, but will appreciate a lot for another review of them.

[annevk]

This generally looks good. I'd like to try this in WebKit, but the patch needs rebasing it seems.

cc @rniwa

[annevk]

Optional nit (applies quite a few times):

Suggested change

	test((test) => {
	test(test => {

[ja-y-son]

@annevk Updated the nit. Let me know if you've tried on Safari and if you think we should make any other changes on the PR