test: fail for missing output files

Instead of ignoring missing `.out` files for message/pseudo-tty tests,
raise an error to indicate that something is not quite right.

Ref: https://github.com/nodejs/node/pull/10037
PR-URL: https://github.com/nodejs/node/pull/10150


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>