Add a test that runs multiple workers that concurrently try to advance
along a random sequence of numbers, using AtomicCompareExchange to
update the shared current position.

Change-Id: Ie073bbdce6fd6766ef1f73f996dd592b90b8b3c2
Reviewed-on: https://chromium-review.googlesource.com/1198769
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55722}