derp, netcheck: make tests listen on localhost only

avoid macOS firewall pop-ups

derp/derp_test.go

@@ -42,7 +42,7 @@ func TestSendRecv(t *testing.T) {
 		clientKeys = append(clientKeys, priv.Public())
 	}
 
-	ln, err := net.Listen("tcp", ":0")
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
 		t.Fatal(err)
 	}