test: use const/let and common.mustCall

remove process.on('exit') because all callbacks are
wrapped by common.mustCall.

PR-URL: https://github.com/nodejs/node/pull/9959


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>