Add image generation test to app route (#46449)

~~Currently blocked by #46448.~~
Closes vercel/satori#410.

## Bug

- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/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`
- [ ]
[e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

test/e2e/og-api/app/app/og/route.js

@@ -0,0 +1,10 @@
+import { ImageResponse } from '@vercel/og'
+
+export async function GET() {
+  return new ImageResponse(<div>hi</div>, {
+    width: 1200,
+    height: 630,
+  })
+}
+
+export const runtime = 'edge'

test/e2e/og-api/app/next.config.js

@@ -1,4 +1,7 @@
 /** @type {import('next').NextConfig} */
 module.exports = {
   output: 'standalone',
+  experimental: {
+    appDir: true,
+  },
 }

test/e2e/og-api/index.test.ts

@@ -22,9 +22,18 @@ describe('og-api', () => {
     expect(html).toContain('hello world')
   })
 
-  it('should work', async () => {
+  it('should work in pages/api', async () => {
     const res = await fetchViaHTTP(next.url, '/api/og')
     expect(res.status).toBe(200)
+    expect(res.headers.get('content-type')).toContain('image/png')
+    const body = await res.blob()
+    expect(body.size).toBeGreaterThan(0)
+  })
+
+  it('should work in app route', async () => {
+    const res = await fetchViaHTTP(next.url, '/og')
+    expect(res.status).toBe(200)
+    expect(res.headers.get('content-type')).toContain('image/png')
     const body = await res.blob()
     expect(body.size).toBeGreaterThan(0)
   })