test: update coverage for dgram.js

Create test-dgram-multicast-loopback.js for dgram.setMulticastLoopback.
Add a test which has an argument of an invalid number to test-dgram-setTTL.js.
https://github.com/nodejs/node/blob/master/lib/dgram.js#L464
Add a test which has an argument of a string to test-dgram-multicast-setTTL.js.
https://github.com/nodejs/node/blob/master/lib/dgram.js#L473

test/parallel/test-dgram-multicast-loopback.js

@@ -0,0 +1,12 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const dgram = require('dgram');
+const socket = dgram.createSocket('udp4');
+
+socket.bind(0);
+socket.on('listening', common.mustCall(() => {
+  const result = socket.setMulticastLoopback(16);
+  assert.strictEqual(result, 16);
+  socket.close();
+}));

test/parallel/test-dgram-multicast-setTTL.js

@@ -1,23 +1,23 @@
 'use strict';
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const dgram = require('dgram');
 const socket = dgram.createSocket('udp4');
-let thrown = false;
 
 socket.bind(0);
-socket.on('listening', function() {
-  socket.setMulticastTTL(16);
+socket.on('listening', common.mustCall(() => {
+  const result = socket.setMulticastTTL(16);
+  assert.strictEqual(result, 16);
 
   //Try to set an invalid TTL (valid ttl is > 0 and < 256)
-  try {
+  assert.throws(() => {
     socket.setMulticastTTL(1000);
-  } catch (e) {
-    thrown = true;
-  }
+  }, /^Error: setMulticastTTL EINVAL$/);
 
-  assert(thrown, 'Setting an invalid multicast TTL should throw some error');
+  assert.throws(() => {
+    socket.setMulticastTTL('foo');
+  }, /^TypeError: Argument must be a number$/);
 
   //close the socket
   socket.close();
-});
+}));

test/parallel/test-dgram-setTTL.js

@@ -1,17 +1,22 @@
 'use strict';
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const dgram = require('dgram');
 const socket = dgram.createSocket('udp4');
 
 socket.bind(0);
-socket.on('listening', function() {
+socket.on('listening', common.mustCall(() => {
   const result = socket.setTTL(16);
   assert.strictEqual(result, 16);
 
-  assert.throws(function() {
+  assert.throws(() => {
     socket.setTTL('foo');
-  }, /Argument must be a number/);
+  }, /^TypeError: Argument must be a number$/);
+
+  // TTL must be a number from > 0 to < 256
+  assert.throws(() => {
+    socket.setTTL(1000);
+  }, /^Error: setTTL EINVAL$/);
 
   socket.close();
-});
+}));