test: add coverage for napi_cancel_async_work

adding test coverage for napi_cancel_async_work based
on coverage report

PR-URL: https://github.com/nodejs/node/pull/12575


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