test: add test for dgram.setTTL

Verify that passing a non-number will throw and that the argument is
returned on success.

PR-URL: #2121
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-dgram-setTTL.js

@@ -0,0 +1,17 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const dgram = require('dgram');
+const socket = dgram.createSocket('udp4');
+
+socket.bind(common.PORT);
+socket.on('listening', function() {
+  var result = socket.setTTL(16);
+  assert.strictEqual(result, 16);
+
+  assert.throws(function() {
+    socket.setTTL('foo');
+  }, /Argument must be a number/);
+
+  socket.close();
+});