dgram,test: add addMembership/dropMembership tests

The only tests for `addMembership()` and `dropMembership()` (from the
`dgram` module) were in `test/internet` which means they almost never
get run. This adds checks in `test/parallel`.

I also did some minor tidying on the existing `test/internet` test.

test/parallel/test-dgram-membership.js

@@ -0,0 +1,87 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const dgram = require('dgram');
+const multicastAddress = '224.0.0.114';
+
+const setup = () => {
+  return dgram.createSocket({type: 'udp4', reuseAddr: true});
+};
+
+// addMembership() on closed socket should throw
+{
+  const socket = setup();
+  socket.close(common.mustCall(() => {
+    assert.throws(() => { socket.addMembership(multicastAddress); },
+                  /Not running/);
+  }));
+}
+
+// dropMembership() on closed socket should throw
+{
+  const socket = setup();
+  socket.close(common.mustCall(() => {
+    assert.throws(() => { socket.dropMembership(multicastAddress); },
+                  /Not running/);
+  }));
+}
+
+// addMembership() with no argument should throw
+{
+  const socket = setup();
+  assert.throws(() => { socket.addMembership(); },
+                /multicast address must be specified/);
+  socket.close();
+}
+
+// dropMembership() with no argument should throw
+{
+  const socket = setup();
+  assert.throws(() => { socket.dropMembership(); },
+                /multicast address must be specified/);
+  socket.close();
+}
+
+// addMembership() with invalid multicast address should throw
+{
+  const socket = setup();
+  assert.throws(() => { socket.addMembership('256.256.256.256'); }, /EINVAL/);
+  socket.close();
+}
+
+// dropMembership() with invalid multicast address should throw
+{
+  const socket = setup();
+  assert.throws(() => { socket.dropMembership('256.256.256.256'); }, /EINVAL/);
+  socket.close();
+}
+
+// addMembership() with valid socket and multicast address should not throw
+{
+  const socket = setup();
+  assert.doesNotThrow(() => { socket.addMembership(multicastAddress); });
+  socket.close();
+}
+
+// dropMembership() without previous addMembership should throw
+{
+  const socket = setup();
+  assert.throws(
+    () => { socket.dropMembership(multicastAddress); },
+    /EADDRNOTAVAIL/
+  );
+  socket.close();
+}
+
+// dropMembership() after addMembership() should not throw
+{
+  const socket = setup();
+  assert.doesNotThrow(
+    () => {
+      socket.addMembership(multicastAddress);
+      socket.dropMembership(multicastAddress);
+    }
+  );
+  socket.close();
+}