go.net/netutil: don't run test on versions of Go prior to 1.3

LGTM=mikioh.mikioh
R=mikioh.mikioh, minux.ma
CC=golang-codereviews
https://golang.org/cl/97970045

netutil/listen_test.go

@@ -2,6 +2,11 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.3
+
+// (We only run this test on Go 1.3 because the HTTP client timeout behavior
+// was bad in previous releases, causing occasional deadlocks.)
+
 package netutil
 
 import (