test: improve test-assert.js

Verify error message thrown from assert.ifError

PR-URL: https://github.com/nodejs/node/pull/11193


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>