cmd/go: print `go test -help` to stderr

Usage of all commands is printed to stderr, except go test, which is printed to
stdout. This is inconsistent.

Print `go test -help` to stderr instead.

R=rsc@golang.org

Change-Id: I079f4788134bf9aedcccc26838879eedad1c925e
Reviewed-on: https://go-review.googlesource.com/15434Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/go/main.go

@@ -285,7 +285,7 @@ func printUsage(w io.Writer) {
 func usage() {
 	// special case "go test -h"
 	if len(os.Args) > 1 && os.Args[1] == "test" {
-		os.Stdout.WriteString(testUsage + "\n\n" +
+		os.Stderr.WriteString(testUsage + "\n\n" +
 			strings.TrimSpace(testFlag1) + "\n\n\t" +
 			strings.TrimSpace(testFlag2) + "\n")
 		os.Exit(2)