Previously, click handling for labels was handled in layout and
painting code. This change implements activation_behavior on
HTMLLabelElement, which clicks and focuses the element.
This recovers 750+ WPT subtests that were lost when
https://github.com/web-platform-tests/wpt/pull/56913 was merged and
added new testcases, two of which exposed this crash.
I have added my own testcase instead of importing the affected WPT tests
since they are large and complex, which makes it hard to understand
where the problem is coming from based on them alone. Also this is only
a crash test (i.e. not a different kind) because the tested scenario
doesn't actually behave correctly yet for seemingly unrelated reasons.
