test: fix multicast join test on OSX when using IPv6

When choosing an IPv6 interface all low level APIs use the interface
index which can be gathered with if_nametoindex. Passing a 0 means to
use the default as per the documentation. On OSX, however, this is not
the case and when 0 is specified it gives EADDRNOTAVAIL error. So, we
workaround the test, by using the 'lo0' interface on OSX, which has an
index of 1.