test: buffer should always be stringified

This test makes sure that independently of the buffer type, the input
is always stringified and generates a valid input.

PR-URL: https://github.com/nodejs/node/pull/12355


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>