Create test for checking markup serialization

I second that, the parent issue also triggered that thinking for me.

We should make one test that covers all cases that are relevant for serialiation, what I can think of is:

CE with attributes, no children
CE with attributes, single slot, single CE child
CE with attributes, single slot, multiple CE children
CE with attributes, single slot, markup cotnent
CE with attributes, single slot with multiple entries, one single child, one multiple CE children. adding markup in this case is unusual, so not something we have to cover imo.

then, we should serialize this in those formats:
markup with vue2/standard CE syntax
markup with vue3 syntax
json

This is not a release blocker, but I'd love to get it done soon to ensure we are good and stay good here. We should make this one or multiple dedicated kernel tests, so we can easily add them to 2.x and 3.x and compare results.

For reference to whomever:

Markup:
See renderCustomElement() in existing tests.

JSON:
$this->getCustomElementNormalizer()->normalize($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.)