test: refactor addons-napi/test_exception/test.js

* provide block scoping to prevent unintended side effects
* remove confusing and unnecessary assertion message
* use consisitent `actual, expected` argument order for assertions

PR-URL: https://github.com/nodejs/node/pull/18340


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>