[RFC] Upgrade the reftest list format

[kvark]

Looking into the growing list of supported prefixes/functions now: https://github.com/servo/webrender/pull/2105/files#diff-f340cf8bfec5772b98e4211803fbe7cdR219

The format chosen ("xxx.list") doesn't seem to scale nicely, given that all the prefixes are expected to come on the same line... so the tests don't align, and the very fact we are using a simple text format falls down here.

I suggest describing the tests in a structured format, e.g. RON:

[
  (
    op: NonEqual("text.yaml", "blank.yaml"), // or we could have src/dst fields
    fuzz: (diff: 1, count: 100),
    aa: false,
    platforms: [Linux],
    draw_calls: 4,
  ),
  (
    op: Equal("decorations.yaml", "decorations-suite.png"),
    aa: true,
  )
]