doc: fix link in the test/README.md

PR-URL: https://github.com/nodejs/node/pull/15642


Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>