Test coverage for markup and JSON serialization

For reference to whomever:

Markup:
See renderCustomElement() in existing tests.

$render = $customElement->toRenderArray();
$output = (string) $this->container->get('renderer')->renderPlain($render);

JSON:
$output = $this->getCustomElementNormalizer()->normalize($custom_element). (There is no place in custom_elements module that does JSON serialization, but the building blocks - i.e. getCustomElementNormalizer() are here so the test should be here. One module that has the above line of code, is lupus_ce_renderer.)

This looks good, and I also learned things about CE in the process. (The outdated issue summary is a sign of how little I knew about CustomElement basics back in September -- I likely won't bother updating it.)

I have some requests for additions though; see MR comments.

Please change ticket status to "Needs review" when I should review.

Also: I will want to have a larger comment on the class (to say that it is not only testing CustomElements but also rendering) and possibly rename it. But I think I'll just do that myself afterwards, and not bother you with that.

adding markup in this case is unusual, so not something we have to cover imo.

I actually think we'll need to cover this because it will become more important. But we can add that coverage in #3494426: Create markup to custom-elements tree parser after merging this.

Reviewed. This is good. But done:

• In https://git.drupalcode.org/project/custom_elements/-/merge_requests/109#..., when I said "single setSlot() call" and "above", I meant the test above my comment that contained a single setSlot() call. Because that would test/document an isolated addSlot() call. You have extended a more complicated test, so... I now added this myself. I respected/took the code setup that you used in other tests.
• testSlotOperationsWithMultipleChildren() did not contain the Vue3 version yet, so I added it for consistency - and changed the test comments a bit.
• I renamed the class, and commented that it actually tests the combination of CustomElement + CustomElementsNormalizer.

I don't usually merge issues that are still set to Needs Work, but review was requested elsewhere and we can continue in #3494426: Create markup to custom-elements tree parser if needed.