Fix DatagramChannelMulticastTest#test_joinAnySource_IPv6

This test works by sending a packet and ensuring that we receive
it. But while we specify the interface to receive on
(specifically: ipv6NetworkInterface), we don't specify which
interface to send the packet on. As a result, the kernel is free
to use whatever interface it wants. When the test fails, the
kernel picks the wrong interface.

Bug: 38167941
Test:  make cts && cts-tradefed run singleCommand cts -a arm64-v8a \
       -m CtsLibcoreTestCases -t libcore.java.nio.channels.DatagramChannelMulticastTest
Change-Id: Ie7d62901adfbf5b37c5ad8a67d299b7b15955ce3
1 file changed