test: fix flaky test-dgram-empty-packet & friends

* Liberal use of common.mustCall()
* Rename test-dgram-empty-packet -> test-dgram-send-empty-packet
* Remove use of timers to avoid CI failures like seen in the Ref below:

```
not ok 237 parallel/test-dgram-empty-packet
  ---
  duration_ms: 0.717
  severity: fail
  stack: |-
        ...
        throw new Error('Timeout');
        ^

    Error: Timeout
        at Timeout._onTimeout
        ...
        at ontimeout (timers.js:365:14)
        at tryOnTimeout (timers.js:237:5)
        at Timer.listOnTimeout (timers.js:207:5)
```

Refs: https://ci.nodejs.org/job/node-test-commit-freebsd/5341/nodes=freebsd11-x64/console:
PR-URL: https://github.com/nodejs/node/pull/9724


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