Add more wpt for events sorting.
Add some tests for animation events sorting, per the spec issue:
w3c/csswg-drafts#11064 (comment),
and follow the spec:
https://drafts.csswg.org/css-animations-2/#animation-composite-order
Differential Revision: https://phabricator.services.mozilla.com/D230610
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1933281
gecko-commit: 495c6765094e8da352e02924e824e5876a64a223
gecko-reviewers: hiro