[css-flex] Fix a test that had bad expectations

Some hand calculations in the reference didn't age well. I replaced the
-ref.html with checkLayout.

The test failed in Firefox in an identical fashion. New test passes in
both browsers.

Just a test, no behavior change.

Fixed: 1111708
Change-Id: I7fc82e5d990c57db6c0d205e30aacaed88990418
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490251
Commit-Queue: David Grogan <dgrogan@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820506}