runtime: reset GOMAXPROCS during tests
runtime: reset GOMAXPROCS during tests

Fix the fact that the test leaves GOMAXPROCS=3
and a running goroutine behind.

R=golang-dev, rsc
CC=dvyukov, golang-dev
http://codereview.appspot.com/4517121

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go