cmd/go: show testflag help for "go test -h"

Fixes #6576.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/14502065

src/cmd/go/main.go

@@ -238,6 +238,11 @@ func printUsage(w io.Writer) {
 }
 
 func usage() {
+	// special case "go test -h"
+	if len(os.Args) > 1 && os.Args[1] == "test" {
+		help([]string{"testflag"})
+		os.Exit(2)
+	}
 	printUsage(os.Stderr)
 	os.Exit(2)
 }