If both isolates run this test, they are kind of racing on the
{tier_down_} flag, hence you get unexpected results.

Bug: v8:10189, v8:10099
Change-Id: I8657c32c024898442d5c916cf15077c846276b1f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2044544
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66188}