test: add dgram.Socket.prototype.sendto's test

Refs: https://github.com/nodejs/node/blob/09ebdf14005cc948529b3f193ad550d5dfded26a/lib/dgram.js#L234
PR-URL: nodejs#10901
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/parallel/test-dgram-sendto.js

@@ -0,0 +1,24 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const dgram = require('dgram');
+const socket = dgram.createSocket('udp4');
+
+const errorMessage =
+  /^Error: Send takes "offset" and "length" as args 2 and 3$/;
+
+assert.throws(() => {
+  socket.sendto();
+}, errorMessage);
+
+assert.throws(() => {
+  socket.sendto('buffer', 1, 'offset', 'port', 'address', 'cb');
+}, errorMessage);
+
+assert.throws(() => {
+  socket.sendto('buffer', 'offset', 1, 'port', 'address', 'cb');
+}, errorMessage);
+
+assert.throws(() => {
+  socket.sendto('buffer', 1, 1, 10, false, 'cb');
+}, /^Error: udp4 sockets must send to port, address$/);