test: refactor parallel/test-tls-pause

Use arrow functions and common.mustCall() and add a description.

PR-URL: https://github.com/nodejs/node/pull/18714


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>