Add basic smoke test for per-segment fetching

We can't test per-segment fetching end-to-end yet because the client
behavior hasn't been implemented. These are basic smoke tests to
demonstrate that the server correctly responds to a request. We will
replace them with proper e2e tests once more of the feature has
been implemented.

test/e2e/app-dir/ppr-navigations/simple/per-segment-prefetching.test.ts

@@ -0,0 +1,46 @@
+import { nextTestSetup } from 'e2e-utils'
+
+describe('per segment prefetching', () => {
+  if ((global as any).isNextDev) {
+    test('ppr is disabled in dev', () => {})
+    return
+  }
+
+  const { next } = nextTestSetup({
+    files: __dirname,
+  })
+
+  // This feature is only partially implemented; the client does not yet issue
+  // these types of requests. This tests that the server responds correctly.
+  // TODO: Replace with e2e tests once more is implemented.
+
+  it('prefetch an individual segment', async () => {
+    const response = await next.fetch('/en', {
+      headers: {
+        RSC: '1',
+        'Next-Router-Prefetch': '1',
+        'Next-Router-Segment-Prefetch': '/',
+      },
+    })
+    expect(response.status).toBe(200)
+    const responseText = await response.text()
+    expect(responseText.trim()).toBe(
+      // The actual data is not yet generated, but this indicates that the
+      // request was handled correctly.
+      'TODO (Per Segment Prefetching): Not yet implemented'
+    )
+  })
+
+  it('respond with 404 if the segment does not have prefetch data', async () => {
+    const response = await next.fetch('/en', {
+      headers: {
+        RSC: '1',
+        'Next-Router-Prefetch': '1',
+        'Next-Router-Segment-Prefetch': '/does-not-exist',
+      },
+    })
+    expect(response.status).toBe(404)
+    const responseText = await response.text()
+    expect(responseText.trim()).toBe('')
+  })
+})