test: fix warning in n-api reference test

Add cast to avoid warning during build of addon.

PR-URL: https://github.com/nodejs/node/pull/12730


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>