next.js/packages/next/src/experimental/testing/server/README.md

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.