light-dark-image test using features that are not supported so the test fails for the wrong reason

[dletorey]

Unless I am missing something this is not truly testing for nesting in light-dark()

This is the PR with the related change.

I have built a few codepens for testing the nesting of image-set() in light-dark()

Example 1 light-dark(image-set())

background-image: light-dark(
  image-set(
    url(light.webp) type"image/webp",
    url(dark.webp) type"image/webp"
  ),
  image-set(
    url(light.png) type"image/png",
    url(dark.png) type"image/png"
  )
)

Example 2 image-set(light-dark())

background-image: image-set(
  light-dark(
    url(light.webp),
    url(dark.webp)
  ) type"image/webp",
  light-dark(
    url(light.png),
    url(dark.png)
  ) type"image/png"
)

Both of these work in Firefox, but this test includes -webkit-cross-fade() which is not, yet supported in Firefox so the test fails.

[dletorey]

cc @CGQAQ