Add integration test of API routes with concurrentFeatures enabled (v…

…ercel#31227)

Ensures that API routes are working as expected with `concurrentFeatures: true`. Closes vercel#30586.

## Bug

- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

test/integration/react-streaming-and-server-components/app/pages/api/ping.js

@@ -0,0 +1,3 @@
+export default (req, res) => {
+  res.end('pong')
+}

test/integration/react-streaming-and-server-components/test/index.test.js

@@ -288,6 +288,11 @@ async function runBasicTests(context) {
     const content = await browser.eval(`window.document.body.innerText`)
     expect(content).toMatchInlineSnapshot('"next_streaming_data"')
   })
+
+  it('should support api routes', async () => {
+    const res = await renderViaHTTP(context.appPort, '/api/ping')
+    expect(res).toContain('pong')
+  })
 }
 
 function runSuite(suiteName, env, { runTests, before, after }) {