selftests: bonding: add ns multicast group testing

Add a test to make sure the backup slaves join correct multicast group
when arp_validate enabled and ns_ip6_target is set. Here is the result:

TEST: arp_validate (active-backup ns_ip6_target arp_validate 0)     [ OK ]
TEST: arp_validate (join mcast group)                               [ OK ]
TEST: arp_validate (active-backup ns_ip6_target arp_validate 1)     [ OK ]
TEST: arp_validate (join mcast group)                               [ OK ]
TEST: arp_validate (active-backup ns_ip6_target arp_validate 2)     [ OK ]
TEST: arp_validate (join mcast group)                               [ OK ]
TEST: arp_validate (active-backup ns_ip6_target arp_validate 3)     [ OK ]
TEST: arp_validate (join mcast group)                               [ OK ]
TEST: arp_validate (active-backup ns_ip6_target arp_validate 4)     [ OK ]
TEST: arp_validate (join mcast group)                               [ OK ]
TEST: arp_validate (active-backup ns_ip6_target arp_validate 5)     [ OK ]
TEST: arp_validate (join mcast group)                               [ OK ]
TEST: arp_validate (active-backup ns_ip6_target arp_validate 6)     [ OK ]
TEST: arp_validate (join mcast group)                               [ OK ]

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>