=doc #15772 Make IPv6 multicast tests work on OS X

* Fixed a small race by waiting for the listenert to be bound
* Changed the Java test to test the Java code (imported the wrong class)
* Changed to a known link local multicas address (the old one just gave me a nop route to host)
Merge pull request #15785 from akka/wip-15772-ipv6-udp-multicast-ban

=doc #15772 Make IPv6 multicast tests work on OS X