The previous change to futex.js will spawn too many workers in d8 in
some test configurations, which will throw an error.

This CL works around that by spawning fewer workers for the
Atomics.notify tests.

TBR=adamk@chromium.org

Bug: v8:7338
Change-Id: I0f3583781e5352b4d6672d43a087dc56d920122d
Reviewed-on: https://chromium-review.googlesource.com/1142895Reviewed-by: Ben Smith <binji@chromium.org>
Commit-Queue: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54536}