test: ensure nextTick is not scheduled in exit

Previously our tests did not check this codepath as seen at
coverage.nodejs.org

PR-URL: https://github.com/nodejs/node/pull/9555


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>