test: fix flaky test-debug-port

It can happen that first data chunk received in stdout is not exactly
`'debug> '`. Make sure the exit condition is met.

PR-URL: https://github.com/nodejs/node/pull/10316


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>