We need to yield to the runtime every now and again to avoid
deadlock. This doesn't show up on most machines because the test
only runs when you have 5 or more CPUs.

Fixes #20072.

Change-Id: Ibf5ed370e919943395f3418487188df0b2be160b
Reviewed-on: https://go-review.googlesource.com/112978
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>