test: fix test-cluster-dgram-1 flakiness

Check for the number of messages received in the `exit` event listener
instead of the `disconnect` listener.

Fixes: https://github.com/nodejs/node/issues/8380
PR-URL: https://github.com/nodejs/node/pull/8383


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed By: James M Snell <jasnell@gmail.com>