part 7: Add a second pass to 'stretch' WPTs to test with "box-sizing:border-box", to validate that it doesn't impact the sizing.
box-sizing:border-box shouldn't impact the sizing because we're growing to fill
the containing block, which means we end up with the same overall size in the
relevant axis regardless of whether we're stretching the border-box or the
content-box.
(There's one special case in these tests where it does actually matter, but
not in a way that's really based on how 'stretch' itself is resolved; so I
simply opt that subtest out of this second pass.)
Differential Revision: https://phabricator.services.mozilla.com/D230297
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1933288
gecko-commit: 7892bfc41933d4fe8b8ca6596f4cb4e50161e4e1
gecko-reviewers: TYLin