test: increase setMulticastLoopback() coverage

PR-URL: #11277
Reviewed-By: James M Snell <jasnell@gmail.com>

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

@@ -2,11 +2,22 @@
 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();
-}));
+
+{
+  const socket = dgram.createSocket('udp4');
+
+  assert.throws(() => {
+    socket.setMulticastLoopback(16);
+  }, /^Error: setMulticastLoopback EBADF$/);
+}
+
+{
+  const socket = dgram.createSocket('udp4');
+
+  socket.bind(0);
+  socket.on('listening', common.mustCall(() => {
+    assert.strictEqual(socket.setMulticastLoopback(16), 16);
+    assert.strictEqual(socket.setMulticastLoopback(0), 0);
+    socket.close();
+  }));
+}