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: #30227 (comment)

PR-URL: #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>

Author: addaleax