[scroll-animations] multiple issues with `animation-timeline` and the `animation` shorthand

While looking at adding support for the various CSS properties relevant to Scroll-linked Animations, I came across these tests which look incorrect to me:

https://github.com/web-platform-tests/wpt/blob/e8d57f52b64b733a0927503ed31b8e4e9c7100a7/css/css-animations/parsing/animation-computed.html#L20-L38

This test does not match the CSS Animations Level 2 grammar for animation-timeline since the final value <single-animation-timeline> is not serialized. I realize that this test is perfectly valid for implementations that support CSS Animations Level 1, but how should we deal with the expansion of the animation shorthand?

https://github.com/web-platform-tests/wpt/blob/e8d57f52b64b733a0927503ed31b8e4e9c7100a7/scroll-animations/css/animation-shorthand.html#L18-L21

This test assumes scroll() and view() are not valid values for animation-timeline but they are. This seems like a straight up bug that I'd be happy to file a PR for.

Start with css/css-animations/parsing/animation-computed.html and scroll-animations/css/animation-shorthand.html at the referenced lines, then compare their expectations with the CSS Animations Level 2 grammar. Update the tests so animation-timeline serialization and scroll()/view() validity match the specification, and run the relevant WPT tests to confirm the corrected behavior.