test: use expectsError in test-debug-agent.js

Use common.expectsError() in place of inline validation function in
test-debug-agent.js.

PR-URL: https://github.com/nodejs/node/pull/11410


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>