test: remove watchdog in test-debug-signal-cluster

test-debug-signal-cluster contains a watchdog timer that results in
false positives in CI. Remove the watchdog timer and let the test runner
determine that the test has timed out.

PR-URL: https://github.com/nodejs/node/pull/9476


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