before   after
go test -short time                4.144s  1.215s
go test -short runtime             1.315s  0.351s
go test -short -cpu=1,2,4 runtime  4.376s  1.266s

Partially solves issue 3015.

R=golang-dev, r
CC=golang-dev, remy
https://golang.org/cl/5673045