test: fix message test after V8 upgrade

With V8 6.2 there is one line less in the promise trace.

PR-URL: https://github.com/nodejs/node/pull/15362
Backport-PR-URL: https://github.com/nodejs/node/pull/16413


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>