selftests: netfilter: conntrack_vrf.sh: add fib test case

meta iifname veth0 ip daddr ... fib daddr oif

... is expected to return "dummy0" interface which is part of same vrf
as veth0.

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