lib,test: minor refactoring

This refactors a couple tests to have upper case first characters
in comments and to use `input` instead of `i`.
It also adds a few TODOs and rewrites a few lines to use default
arguments and to prevent function recreation when unnecessary.

PR-URL: https://github.com/nodejs/node/pull/19445


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>