selftests: netfilter: add fib test case

There is a bug report on netfilter.org bugzilla pointing to fib
expression dropping ipv6 DAD packets.

Add a test case that demonstrates this problem.

Next patch excludes icmpv6 packets coming from any to linklocal.

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