test: changed function to arrow function

Convert callback functions that are anonymous
to arrow functions for better readability.
Also adjusted the `this` object in places
where that was required.

PR-URL: https://github.com/nodejs/node/pull/28726


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>