test: improve util inspect tests

Remove unnecessary code parts and outdated code

PR-URL: https://github.com/nodejs/node/pull/14881


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>