Fuzzers are executed in their own process, so instead of resetting flags
after execution, we can just keep the flag values.
This CL introduces a shared function to enable all staged features,
without ever resetting the value. This fixes a data race.

R=ahaas@chromium.org

Bug: v8:10979
Change-Id: I82ea35b887841850edd8b394a3644cf8df1e3bf8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2449969
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70320}