[css-fonts] Add rendering tests for font palettes

[litherum]

This is an export of https://bugs.webkit.org/show_bug.cgi?id=230792.

I didn't test the <string> value for base-palette or override-colors
because w3c/csswg-drafts#6627 isn't resolved
yet.

The test font included in this patch doesn't have any named palettes
or colors because I currently don't have any way of testing that
(not even in native code). So I wouldn't have been able to know if
the font was constructed correctly.

There are 2 fonts included in this patch: Ahem-COLR-palettes.ttf
and Ahem-COLR-palettes-no-light-dark.ttf. Ahem-COLR-palettes.ttf is a
COLR font with 64 colored characters, each with 2 component glyphs.
The 2 components hold every combination of 8 colors (to make a total
of 64 colored glyphs). There are also 16 palettes of those 8 colors,
arranged in a rainbow table. Palette 2 is marked as
USABLE_WITH_LIGHT_BACKGROUND and palette 3 is marked as
USABLE_WITH_DARK_BACKGROUND. Ahem-COLR-palettes-no-light-dark.ttf
is much simpler, only has a single color character, and 2 palettes
of 2 colors each, and none of the palettes are marked as
USABLE_WITH_LIGHT_BACKGROUND or USABLE_WITH_DARK_BACKGROUND.

The fonts were generated from
https://github.com/litherum/AhemMaker/tree/COLR-multiple-palettes.

[drott]

It might make sense to add reftest-wait logic that completes on the document.fonts.ready promis resolution for ensuring font loading completes and avoiding flakiness. We did have a problem with that in the past on our bots but I am not 100% sure to what extent that's still the case.

So I wouldn't have been able to know if the font was constructed correctly.

FWIW, checking whether the font is contructed right with a name array can be done with fonttools. But I am not opposed to removing the <string> value from palette selection with <base-palette>.

[litherum]

I think reftest-wait makes sense for mutation tests, but for static tests like these, if the test runner doesn't wait for static @font-face fonts to be loaded, we have bigger problems than just these new tests. Tons of tests in WPT use (regular normal vanilla) Ahem, and that Ahem is also loaded as a web font.

[litherum]

After talking with @youennf, we think the most straightforward way to make the linter pass is to just rename the new font.