test(tuic): retry until one port is free for both TCP and UDP

The route marker took a free TCP port and bound UDP on the same number, which
fails whenever that UDP port is in use or reserved (Windows excludes port ranges
per protocol), so the TUIC routing tests failed at random on such hosts.

internal/web/service/tuic_user_routing_test.go

@@ -272,17 +272,28 @@ func TestTuicUserRoutingDoesNotDoubleCountTraffic(t *testing.T) {
 	}
 }
 
+// listenTCPAndUDPOnOnePort retries because a free TCP port may be taken or reserved for UDP
+// (Windows excludes port ranges per protocol).
+func listenTCPAndUDPOnOnePort(t *testing.T) (net.Listener, net.PacketConn) {
+	t.Helper()
+	for range 20 {
+		listener, err := net.Listen("tcp", "127.0.0.1:0")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if packet, err := net.ListenPacket("udp", listener.Addr().String()); err == nil {
+			return listener, packet
+		}
+		_ = listener.Close()
+	}
+	t.Fatal("no port free for both TCP and UDP")
+	return nil, nil
+}
+
 func startTuicRouteMarker(t *testing.T, marker string) string {
 	t.Helper()
-	listener, err := net.Listen("tcp", "127.0.0.1:0")
-	if err != nil {
-		t.Fatal(err)
-	}
+	listener, packet := listenTCPAndUDPOnOnePort(t)
 	t.Cleanup(func() { _ = listener.Close() })
-	packet, err := net.ListenPacket("udp", listener.Addr().String())
-	if err != nil {
-		t.Fatal(err)
-	}
 	t.Cleanup(func() { _ = packet.Close() })
 	go func() {
 		for {