test: skip IPv6 test on non-IPv6 systems

Until recently, test-dgram-address would fail on machines without IPv6
but still exit with a successful return code. (It would console.log()
the error but not actually fail.)

Now that the test has been updated such that it will fail the IPv6 part
of the test if there is an error event emitted by the socket, skip the
test on systems not equipped with IPv6.

PR-URL: nodejs#11432
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Rod Vagg <rod@vagg.org>

test/parallel/test-dgram-address.js

@@ -26,7 +26,7 @@ const dgram = require('dgram');
   socket.bind(0, common.localhostIPv4);
 }
 
-{
+if (common.hasIPv6) {
   // IPv6 Test
   const socket = dgram.createSocket('udp6');
   const localhost = '::1';