web-platform-tests/wpt merge_pr_47251

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Add the tentative serialization for animation shorthand.

In order to avoid this unexpected fail, we add this tentative
serialization for animation shorthand.

1. animation-timeline is reset-only in this shorthand, so we don't parse
   it and we reset it to vec![auto].
2. we don't serialize the shorthand if animation-timeline is not the initial
   value.

This is a tentative solution and we should update it with the new syntax
once the spec gets update (w3c/csswg-drafts#6946).

Differential Revision: https://phabricator.services.mozilla.com/D217164

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1908819
gecko-commit: 4f6324b24c80d4fdf4d825d009ec84d9c76e85d7
gecko-reviewers: firefox-style-system-reviewers, layout-reviewers, emilio