test: add logging for test-debug-port-cluster

The test is currently flaky and CI provides no real information because
the test times out rather than failing on an assertion. Add logging to
gather more information about the failure.

Refs: https://github.com/nodejs/node/issues/6754
PR-URL: https://github.com/nodejs/node/pull/6769


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>