n-api: update reference test

Remove the necessity for allocating on the heap, and assert that the
correct pointer gets passed to the finalizer.

PR-URL: https://github.com/nodejs/node/pull/19086


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>