test: remove AtExit() addon test

Move the one bit of the addon test that was not covered by the
cctest into the latter and delete the former.

Refs: https://github.com/nodejs/node/pull/30227#discussion_r341879242

PR-URL: https://github.com/nodejs/node/pull/30275


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>