test: fix flaky test-cluster-dgram-2

There is no guarantee that a dgram packet will be received. The test is
currently written to only send exactly as many dgram packets as required
assuming they are all received. As a result, failures like this may
occur (from CI):

```
not ok 719 parallel/test-cluster-dgram-2
  ---
  duration_ms: 120.39
  severity: fail
  stack: |-
    timeout
```

This change has the workers send packets continuously until disconnect.

PR-URL: https://github.com/nodejs/node/pull/9791


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>