selftests: netfilter: add ebtables broute test case

ebtables -t broute allows to redirect packets in a way that
they get pushed up the stack, even if the interface is part
of a bridge.

In case of IP packets to non-local address, this means
those IP packets are routed instead of bridged-forwarded, just
as if the bridge would not have existed.

Expected test output is:
PASS: netns connectivity: ns1 and ns2 can reach each other
PASS: ns1/ns2 connectivity with active broute rule
PASS: ns1/ns2 connectivity with active broute rule and bridge forward drop

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>