RFR: 8217427: [TESTBUG] nio/channels/DatagramChannel/UseDGWithIPv6.java fails without IPv6

[Nick Gasson (Arm Technology China)]

Hi,

Please review this small change to skip the above test if run on a 
system with IPv6 disabled.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217427
Webrev: http://cr.openjdk.java.net/~ngasson/8217427/webrev.01/

The convention in other tests (e.g. 
java/net/DatagramSocket/LocalSocketAddress.java) is to skip the test if 
IPv6 is disabled, but UseDGWithIPv6.java will fail with an 
UnsupportedAddressException.

Also tested on a system with IPv6 enabled to check the test still runs.

Thanks,
Nick