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/15434
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/src/cmd/go/main.go b/src/cmd/go/main.go
index ef035c7..b66049f 100644
--- a/src/cmd/go/main.go
+++ b/src/cmd/go/main.go
@@ -285,7 +285,7 @@
 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)