net: make newLocalListener handle network argument correcly

Change-Id: I8987e705af069846e6668e2f2104e0254e695139
Reviewed-on: https://go-review.googlesource.com/19706
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/mockserver_test.go

@@ -30,10 +30,20 @@ func testUnixAddr() string {
 
 func newLocalListener(network string) (Listener, error) {
 	switch network {
-	case "tcp", "tcp4", "tcp6":
+	case "tcp":
+		if supportsIPv4 {
+			if ln, err := Listen("tcp4", "127.0.0.1:0"); err == nil {
+				return ln, nil
+			}
+		}
+		if supportsIPv6 {
+			return Listen("tcp6", "[::1]:0")
+		}
+	case "tcp4":
 		if supportsIPv4 {
 			return Listen("tcp4", "127.0.0.1:0")
 		}
+	case "tcp6":
 		if supportsIPv6 {
 			return Listen("tcp6", "[::1]:0")
 		}