They are not transitionable:
https://drafts.csswg.org/css-transitions-1/#transitionable
There are some new failures in background-image-interpolation.html,
but I think the test is wrong, because it expects background-image
to be transitionable, even though the spec defines it with a discrete
animation type.
* Update web-platform-tests to revision b'e16853520ae70ba309108ccff87a7816f74be4dd'
* Remove new spurious result
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Co-authored-by: Martin Robinson <mrobinson@igalia.com>