test: move getTTYfd() to common helpers

This utility is fairly generic and likely useful for more than one test.

PR-URL: https://github.com/nodejs/node/pull/18800


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>