test: add cases for unescape & unescapeBuffer

These two functions in the querystring are used as a fallback.
To test them, two test cases were added which make errors that
will be caught.

PR-URL: https://github.com/nodejs/node/pull/11326


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>