test: add test description to fs.readFile tests

PR-URL: https://github.com/nodejs/node/pull/17610
Refs: https://github.com/nodejs/node/pull/17054#discussion_r155406755


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>