8386595: Two java/net/MulticastSocket tests fail on machines with no ipv6 address other than loopback

[ArnoZeller]

The tests java/net/MulticastSocket/Test.java and java/net/MulticastSocket/NoLoopbackPackets.java fail on a Linux server with IPv6 enabled for loopback interface but not for another interface.
They better check not only for any available IPv6 interface, but ensure that it is not only the loopback interface. The join relys on routing and the loopback interface cannot be used for routing as far as I understand.

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Issue

• JDK-8386595: Two java/net/MulticastSocket tests fail on machines with no ipv6 address other than loopback (Bug - P4)

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