net: add skip message to test

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5753048
diff --git a/src/pkg/net/udp_test.go b/src/pkg/net/udp_test.go
index ea5fad4..f80d3b5 100644
--- a/src/pkg/net/udp_test.go
+++ b/src/pkg/net/udp_test.go
@@ -10,7 +10,9 @@
 )
 
 func TestWriteToUDP(t *testing.T) {
-	if runtime.GOOS == "plan9" {
+	switch runtime.GOOS {
+	case "plan9":
+		t.Logf("skipping test on %q", runtime.GOOS)
 		return
 	}