doc: format markdown files in test directory

Use `make format-md` to format markdown files in `test` directory.

PR-URL: https://github.com/nodejs/node/pull/40290


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>