test: add coverage for dgram send() errors

This commit adds code coverage for emitted and callback errors
for dgram's Socket#send() method.

PR-URL: nodejs#11248
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/parallel/test-dgram-send-error.js

@@ -0,0 +1,37 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const dgram = require('dgram');
+const mockError = new Error('mock DNS error');
+
+function getSocket(callback) {
+  const socket = dgram.createSocket('udp4');
+
+  socket.on('message', common.mustNotCall('Should not receive any messages.'));
+  socket.bind(common.mustCall(() => {
+    socket._handle.lookup = function(address, callback) {
+      process.nextTick(callback, mockError);
+    };
+
+    callback(socket);
+  }));
+  return socket;
+}
+
+getSocket((socket) => {
+  socket.on('error', common.mustCall((err) => {
+    socket.close();
+    assert.strictEqual(err, mockError);
+  }));
+
+  socket.send('foo', socket.address().port, 'localhost');
+});
+
+getSocket((socket) => {
+  const callback = common.mustCall((err) => {
+    socket.close();
+    assert.strictEqual(err, mockError);
+  });
+
+  socket.send('foo', socket.address().port, 'localhost', callback);
+});