test: fix test-cluster-dgram-1 flakiness

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test

Description of change

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