Add integration test for Suspense and streaming (#31197)

Adds a test to ensure that `<Suspense>` and streaming are working properly with `concurrentFeatures` enabled.

## 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/streaming.js

@@ -0,0 +1,23 @@
+import { Suspense } from 'react'
+
+let result
+let promise
+function Data() {
+  if (result) return result
+  if (!promise)
+    promise = new Promise((res) => {
+      setTimeout(() => {
+        result = 'next_streaming_data'
+        res()
+      }, 500)
+    })
+  throw promise
+}
+
+export default function Page() {
+  return (
+    <Suspense fallback="next_streaming_fallback">
+      <Data />
+    </Suspense>
+  )
+}

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

@@ -253,6 +253,40 @@ async function runBasicTests(context) {
     expect(html).toContain('bar.server.js:')
     expect(html).toContain('foo.client')
   })
+
+  it('should support streaming', async () => {
+    await fetchViaHTTP(context.appPort, '/streaming', null, {}).then(
+      async (response) => {
+        let result = ''
+        let gotFallback = false
+        let gotData = false
+
+        await new Promise((resolve) => {
+          response.body.on('data', (chunk) => {
+            result += chunk.toString()
+
+            gotData = result.includes('next_streaming_data')
+            if (!gotFallback) {
+              gotFallback = result.includes('next_streaming_fallback')
+              if (gotFallback) {
+                expect(gotData).toBe(false)
+              }
+            }
+          })
+
+          response.body.on('end', () => resolve())
+        })
+
+        expect(gotFallback).toBe(true)
+        expect(gotData).toBe(true)
+      }
+    )
+
+    // Should end up with "next_streaming_data".
+    const browser = await webdriver(context.appPort, '/streaming')
+    const content = await browser.eval(`window.document.body.innerText`)
+    expect(content).toMatchInlineSnapshot('"next_streaming_data"')
+  })
 }
 
 function runSuite(suiteName, env, { runTests, before, after }) {