UnreferencedXXX tests fail when run with --illegal-access=deny

test/jdk/java/net/DatagramSocket/UnreferencedDatagramSockets.java and
test/jdk/java/net/MulticastSocket/UnreferencedMulticastSockets.java need to be fixed in advance of disallowing illegal access by default.

These tests hack into private members such as DatagramSocket.delegate, DatagramChannelImpl.fd so so need to have their test description updated with `@modules java.base/java.net:+open sun.nio.ch:+open`.