test: stricten readline keypress failure test condition

This verifies that the thrown error is the expected one. That was
not tested before.

PR-URL: https://github.com/nodejs/node/pull/31300


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>