Test change proposal: mark light-dark() and system colors as not computationally independent

[zrhoffman]

Test List

• https://wpt.fyi/results/css/css-properties-values-api/at-property.html?label=master&label=experimental&aligned&view=interop&q=label%3Ainterop-2023-property
• https://wpt.fyi/results/css/css-properties-values-api/register-property-syntax-parsing.html?label=master&label=experimental&aligned&view=interop&q=label%3Ainterop-2023-property

The additions are in https://phabricator.services.mozilla.com/D226869.

Rationale

Both light-dark() and system colors are color-scheme dependent.

From the spec:

A property value is computationally independent if it can be converted into a computed value using only the value of the property on the element, and "global" information that cannot be changed by CSS.

By that definition, colors that are color-scheme dependent are not computationally independent, so they should not be allowed as custom property initial values. I have added test coverage for this in the linked patch.

@dshin-moz has filed bugs for this:

• https://bugs.webkit.org/show_bug.cgi?id=282057
• https://issues.chromium.org/issues/375395106
• https://bugzilla.mozilla.org/show_bug.cgi?id=1926969

cc @nt1m, @andruud, @emilio