netinet6 tests: test for loss of Solicited-node multicast groups

The multicast code has an issue where it can lose the Solicited-node
multicast group subscription if the same address is added twice.

Test for this.

PR: 233683
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D41123