These test mainly the data structures of JSWeakFactory / JSWeakCell.

BUG=v8:8179

Change-Id: I20ffd07c18bbb2e21c69d11aa65d1e245203cc82
Reviewed-on: https://chromium-review.googlesource.com/c/1267939
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56569}