Also skip slowest test on the slowest extra variant.

TBR=sigurds@chromium.org

Bug: v8:7783
Change-Id: I565fa8edd9c3f20fe15af84e1b9023450ef4593e
Reviewed-on: https://chromium-review.googlesource.com/1170832
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55053}