add integration test with mock server

mock_server.ts

@@ -0,0 +1,14 @@
+import { createSocket, RemoteInfo } from "node:dgram";
+export const mockServer = (onMessage: (msg: DataView) => Uint8Array[]) => {
+  const sock = createSocket("udp4");
+  const SEND_PORT = 32108;
+  sock.bind(SEND_PORT);
+  sock.on("message", (msg, rinfo: RemoteInfo) => {
+    const dv = new DataView(new Uint8Array(msg).buffer);
+    onMessage(dv).forEach((out) => {
+      sock.send(out, rinfo.port, rinfo.address);
+    });
+  });
+
+  return sock;
+};