test: fix cluster-disconnect-handles flakiness

Sometimes the test was timing out because the worker process remained
stuck in the breakpoint and didn't exit. This could happen because the
continue was sent before the breakpoint was set. If that's the case,
with this change, a new continue command is sent so the worker process
can end.

PR-URL: https://github.com/nodejs/node/pull/4009


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>