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

* remove custom messages for assert that conceal values
* add comment explaining test
* add block scoping

Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/16814


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>