The number of CPUs is of value when benchmarking but mostly
noise when testing. The recent change to default to the number
of CPUs available has made the tests noisier and confusing.

Fixes #11200

Change-Id: Ifc87d9ccb4177d73e304fb7ffcef4367bd163c9e
Reviewed-on: https://go-review.googlesource.com/11121Reviewed-by: Russ Cox <rsc@golang.org>