next/experimental/testing/server
This directory contains helpers for unit testing Next.js server code, such as
routing using next.config.js or middleware. These utilities can be used to
verify the behavior of redirects, rewrites, adding headers, or middleware logic
before code reaches to production.
See https://nextjs.org/docs/app/building-your-application/routing/middleware and https://nextjs.org/docs/app/api-reference/next-config-js for more details.