test: improve test-buffer-includes.js

* verify error message
* use arrow funcs

PR-URL: https://github.com/nodejs/node/pull/11203


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>