test: fix race condition in addon test

PR-URL: https://github.com/nodejs/node/pull/16037


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>