test: add ut of chile process pod in StdioClientTransport

Author: XiaofuHuang

src/client/stdio.test.ts

@@ -59,3 +59,12 @@ test("should read messages", async () => {
 
   await client.close();
 });
+
+test("should return child process pid", async () => {
+  const client = new StdioClientTransport(serverParameters);
+
+  await client.start();
+  expect(client.pid).toBeDefined();
+  await client.close();
+  expect(client.pid).toBeUndefined();
+});