selftests: netfilter: add bridge conntrack + multicast test case

Add test case for multicast packet confirm race.
Without preceding patch, this should result in:

 WARNING: CPU: 0 PID: 38 at net/netfilter/nf_conntrack_core.c:1198 __nf_conntrack_confirm+0x3ed/0x5f0
 Workqueue: events_unbound macvlan_process_broadcast
 RIP: 0010:__nf_conntrack_confirm+0x3ed/0x5f0
  ? __nf_conntrack_confirm+0x3ed/0x5f0
  nf_confirm+0x2ad/0x2d0
  nf_hook_slow+0x36/0xd0
  ip_local_deliver+0xce/0x110
  __netif_receive_skb_one_core+0x4f/0x70
  process_backlog+0x8c/0x130
  [..]

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>