This is to prevent errors like:
https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Arm/8792

The error code suggests the OS killed the tests due to OOM. The time it took
suggests OS paging. Fewer tests in parallel should mitigate this.

NOTRY=true

Change-Id: I847058cfb02a9a36795581df47760d921d695141
Reviewed-on: https://chromium-review.googlesource.com/c/1333674
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57486}