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

(cherry picked from commit b03012d0b600793d7501b4cc56757ec6150ec87f)