[scroll-animations] Timeline-scope tests should not accept non-dashed idents

[nmoucht]

Regarding tests css/timeline-scope-parsing.tentative.html and css/timeline-scope-computed.tentative.html, both these tests expect normal custom idents to be parsed, despite timeline-scope being defined as none | all | <dashed-ident># (https://drafts.csswg.org/scroll-animations-1/#propdef-timeline-scope).

[nmoucht]

CC @kevers-google, @andruud and @flackr.

[graouts]

This being addressed in WebKit/WebKit#40361 and #50612.

[graouts]

Fixed in 26b440e.