cmd/go: show testflag help for "go test -h" Fixes issue 6576.
cmd/go: show testflag help for "go test -h"
Fixes issue 6576.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/14502065

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang