[test] Cover external `next/image` in adapter deployments (#99466)
The existing `image-from-node-modules` fixture renders `next/image` when
Next.js prerenders its static page. It does not render the external
image component during an SSR request, when an adapter deployment loads
image configuration from the frozen `required-server-files.json`
manifest. This change adds a `getServerSideProps` route, keeps
`my-cool-image` external, and configures two `images.deviceSizes`.
Before #99394, the adapter deployment fails when `next/image` sorts the
frozen manifest configuration.