An IGMPv3 test case, and an MP-BGP nexthop fix

I sent in an IGMPv3 fix forever ago, and you asked me to write a test.  I wrote that test but seem to have forgotten to send you an update.

Meanwhile, I also fixed the printing of multiple MPBGP nexthops (e.g., IPv6 global and link-local) and included a test for that too.