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>

test/parallel/test-dgram-empty-packet.js

@@ -1,40 +0,0 @@
-'use strict';
-const common = require('../common');
-const dgram = require('dgram');
-
-let callbacks = 0;
-let timer;
-
-if (common.isOSX) {
-  common.skip('because of 17894467 Apple bug');
-  return;
-}
-
-const client = dgram.createSocket('udp4');
-
-client.bind(0, function() {
-
-  function callback() {
-    callbacks++;
-    if (callbacks === 2) {
-      clearTimeout(timer);
-      client.close();
-    } else if (callbacks > 2) {
-      throw new Error('the callbacks should be called only two times');
-    }
-  }
-
-  client.on('message', function(buffer, bytes) {
-    callback();
-  });
-
-  const port = this.address().port;
-  client.send(
-    Buffer.allocUnsafe(1), 0, 0, port, '127.0.0.1', (err, len) => {
-      callback();
-    });
-
-  timer = setTimeout(function() {
-    throw new Error('Timeout');
-  }, 200);
-});

test/parallel/test-dgram-send-empty-array.js

@@ -1,24 +1,30 @@
 'use strict';
 
 const common = require('../common');
-const assert = require('assert');
-const dgram = require('dgram');
 
 if (common.isOSX) {
   common.skip('because of 17894467 Apple bug');
   return;
 }
 
+const assert = require('assert');
+const dgram = require('dgram');
+
 const client = dgram.createSocket('udp4');
 
+var interval;
+
 client.on('message', common.mustCall(function onMessage(buf, info) {
   const expected = Buffer.alloc(0);
   assert.ok(buf.equals(expected), 'message was received correctly');
+  clearInterval(interval);
   client.close();
 }));
 
-client.on('listening', function() {
-  client.send([], this.address().port, common.localhostIPv4);
-});
+client.on('listening', common.mustCall(function() {
+  interval = setInterval(function() {
+    client.send([], client.address().port, common.localhostIPv4);
+  }, 10);
+}));
 
 client.bind(0);

test/parallel/test-dgram-send-empty-buffer.js

@@ -1,26 +1,27 @@
 'use strict';
 const common = require('../common');
-const dgram = require('dgram');
+const assert = require('assert');
 
 if (common.isOSX) {
   common.skip('because of 17894467 Apple bug');
   return;
 }
 
+const dgram = require('dgram');
+
 const client = dgram.createSocket('udp4');
 
-client.bind(0, function() {
+client.bind(0, common.mustCall(function() {
   const port = this.address().port;
 
-  client.on('message', common.mustCall(function onMessage(buffer, bytes) {
-    clearTimeout(timer);
+  client.on('message', common.mustCall(function onMessage(buffer) {
+    assert.strictEqual(buffer.length, 0);
+    clearInterval(interval);
     client.close();
   }));
 
   const buf = Buffer.alloc(0);
-  client.send(buf, 0, 0, port, '127.0.0.1', function(err, len) { });
-
-  const timer = setTimeout(function() {
-    throw new Error('Timeout');
-  }, common.platformTimeout(200));
-});
+  var interval = setInterval(function() {
+    client.send(buf, 0, 0, port, '127.0.0.1', common.mustCall(function() {}));
+  }, 10);
+}));

test/parallel/test-dgram-send-empty-packet.js

@@ -0,0 +1,34 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+if (common.isOSX) {
+  common.skip('because of 17894467 Apple bug');
+  return;
+}
+
+const dgram = require('dgram');
+
+const client = dgram.createSocket('udp4');
+
+client.bind(0, common.mustCall(function() {
+
+  client.on('message', common.mustCall(callback));
+
+  const port = this.address().port;
+  const buf = Buffer.alloc(1);
+
+  const interval = setInterval(function() {
+    client.send(buf, 0, 0, port, '127.0.0.1', common.mustCall(callback));
+  }, 10);
+
+  function callback(firstArg) {
+    // If client.send() callback, firstArg should be null.
+    // If client.on('message') listener, firstArg should be a 0-length buffer.
+    if (firstArg instanceof Buffer) {
+      assert.strictEqual(firstArg.length, 0);
+      clearInterval(interval);
+      client.close();
+    }
+  }
+}));