test: fix flaky test-debug-prompt

Be sure to send `.exit` only once to avoid spurious EPIPE and possibly
other errors.

Fixes: https://github.com/nodejs/node/issues/21724

PR-URL: https://github.com/nodejs/node/pull/21826


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>