[layout] More 'stretch' keyword tests
Just some wpt tests for auto margins and indefinite available space.
No behavior changes.
Change-Id: I3f8bd0b769646db3d16f6672b1ab131c97f1a420
Bug: 41253915
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5983888
Reviewed-by: David Baron dbaron@chromium.org
Commit-Queue: David Grogan dgrogan@chromium.org
Cr-Commit-Position: refs/heads/main@{#1377013}