Add tests for CSS Color 5 color-mix()

[weinig]

Tests for parsing and serializing color-mix().

[nt1m]

@emilio might be a good reviewer here since he worked on color-mix in Gecko.

[nt1m]

This is the test Gecko contributed: https://github.com/web-platform-tests/wpt/tree/master/css/css-color/color-mix-basic-001.tentative.html

[weinig]

Because I forgot to mention it in the description, this is a wpt-ified version of the color-mix() tests in WebKit's layout tests that I wrote, https://github.com/WebKit/WebKit/blob/main/LayoutTests/fast/css/parsing-color-mix.html.

[svgeesus]

@nt1m

This is the test Gecko contributed: https://github.com/web-platform-tests/wpt/tree/master/css/css-color/color-mix-basic-001.tentative.html

That test should no longer be marked tentative. It references w3c/csswg-drafts#6066 which is closed, and the test case is conformant to the way the issue was resolved.

[nt1m]

@svgeesus #32064