test: refactor callback functions to arrow functions

Refactor callback functions to modern arrow functions.

Also, added `common.mustCall` to `online` callbacks.

PR-URL: https://github.com/nodejs/node/pull/23546


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>