test-dns-ipv6.js fails on systems without IPv6 support

Hello,

The test should be skipped on systems without IPv6. The same behavior can be found in
[test-net-server-address.js](https://github.com/nodejs/node/blob/master/test/sequential/test-net-server-address.js)

```
if (!common.hasIPv6) {
  console.log('1..0 # Skipped: ipv6 part of test, no IPv6 support');
  return;
}
```
