selftests: netfilter: add test for br_netfilter+conntrack+queue combination

Trigger cloned skbs leaving softirq protection.
This triggers splat without the preceeding change
("netfilter: nf_queue: drop packets with cloned unconfirmed
 conntracks"):

WARNING: at net/netfilter/nf_conntrack_core.c:1198 __nf_conntrack_confirm..

because local delivery and forwarding will race for confirmation.

Based on a reproducer script from Yi Chen.

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