Add a @starting-style subtest for after-change style inheritance across ancestors

[lilles]

There is a subtest for @starting-style that tests that transitions are based on the after-change style, which is supposed to not include animation effects when inheriting from the parent:

https://wpt.fyi/results/css/css-transitions/starting-style-cascade.html?label=master&label=experimental&aligned&view=interop&q=label%3Ainterop-2024-starting-style%20or%20label%3Ainterop-2024-transition-behavior

Firefox is the only browser that implements this correctly, but Safari passes because they inherit correctly if the animating element is a direct parent in the case of @starting-style.

I would like to either get better coverage by adding a subtest (see CL below), or remove the current subtest from interop. Firefox passes this new tests while Chrome and Safari fails.

https://chromium-review.googlesource.com/c/chromium/src/+/5577020

@emilio @nt1m @josepharhar

[emilio]

Sgtm, cc @BorisChiou

[BorisChiou]

The subtest looks good to me.

[foolip]

@gsnedders @nt1m @stubbornella can you take a look at this for WebKit?

[nt1m]

Seems fine! Sorry for the delay, I was out for the last two weeks.

[foolip]

Thanks for review everyone. Closing this, @lilles please go ahead and make the change.

[lilles]

https://github.com/web-platform-tests/wpt/pull/46688