testsuite: Add crossfade-clip-both-children test

This one tests a crossfade between two non-overlapping nodes with a clip
region that covers neither of the two nodes.

This tests that renderers can deal with clip regions that doesn't
overlap nodes in a situation where they will most likely want to create
an offscreen.
As offscreens are typically clipped to the clip region, this would cause
an empty offscreen and that can cause failures.

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