Add reftests for the image(<color>) CSS function

The existing image-function parsing tests cover syntax but not
rendering. Add identity reftests confirming image(<color>) rasterizes
the same as the equivalent background-color, including currentcolor
resolution, multi-layer backgrounds, and use inside cross-fade().

This CL is a follow-up CL of https://chromium-review.googlesource.com/c/chromium/src/+/7831826

Bug: 510426954
Change-Id: I47170665fa5e25dd3267954eaab0d5296c07c886
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7842472
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Jason Leo <cgqaq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1632252}