test: minor refactoring

Add punctuation and comments about code that should not throw.
Also remove a obsolete test and refactor some tests.

PR-URL: https://github.com/nodejs/node/pull/18669


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>