n-api: resolve promise in test

The last promise created by the test for the purposes of making sure
that its type is indeed a promise needs to be resolved so as to avoid
having it left in the pending state at the end of the test.

PR-URL: https://github.com/nodejs/node/pull/19245


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