test: update arrow function style

This commit applies new arrow function linting rules across the
codebase. As it turns out, the only offenders were in the test
directory.

PR-URL: https://github.com/nodejs/node/pull/4813


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>