test: add test-debugger-breakpoint-exists

This adds test coverage to `unpackError()` in
`lib/internal/debugger/inspect_client.js`. That function previously was
untested.

PR-URL: https://github.com/nodejs/node/pull/39570
Refs: https://github.com/nodejs/node-inspect/issues/101


Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>