This deletes the outdated testing example, as it has not been updated to
Payload v3 yet.
We can add it back in the future once we updated it. Generally, the
Next.js testing docs should now be valid:
https://nextjs.org/docs/app/building-your-application/testing. However,
it might still make sense to provide a v3 testing example to showcase
things like booting up an in-memory db
5ba477b810