test: add cmpout to testlib

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5699060
diff --git a/test/helloworld.go b/test/helloworld.go
index 9c33cab..5025ec9 100644
--- a/test/helloworld.go
+++ b/test/helloworld.go
@@ -1,4 +1,4 @@
-// $G $F.go && $L $F.$A && ./$A.out 2>&1 | cmp - $D/$F.out
+// cmpout
 
 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style