Flag reads from background threads are unfortunately scattered and
hard to completely avoid in the current state of V8. An example TSAN
failure:

ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux64%20TSAN/36369/overview

The root cause is that FlagScope destruction modifies flag values on
the main thread, racing with flag reads from the background thread. In
cctests, there's no need to reset flags back to initial values at the
end of tests. Let's simply remove the problematic flag scopes.

Bug: v8:11658
Change-Id: I59ed3794ddc9ed570772726a423dc22afc4dc207
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2843346
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74082}