test: replace anonymous closure functions with arrow functions

In `test/parallel/test-fs-truncate-fd.js`, callbacks use anonymous
closure functions. It is safe to replace them with arrow functions since
these callbacks don't alter their context (`this`). This results in
shorter functions.

PR-URL: https://github.com/nodejs/node/pull/24478


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>