test: check that this != new.target in addon

Add two checks that are there for expository reasons as much as they are
sanity checks.

PR-URL: https://github.com/nodejs/node/pull/15681
Refs: https://github.com/nodejs/node-addon-api/issues/142


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