n-api: test uint32 truncation

Re: https://github.com/nodejs/abi-stable-node/issues/55#issuecomment-403382424
PR-URL: https://github.com/nodejs/node/pull/21722


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