Reduce the number of workers in the atomic compare-exchange test to
make it run faster on systems with few cores.

Bug: v8:8164
Change-Id: I4de22863e1f2a44d9da4416161d08f5acfcece22
Reviewed-on: https://chromium-review.googlesource.com/1224771
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55887}