selftests: netfilter: add meta iif/oif match test

simple test case, but would have caught this:

FAIL: iifgroupcount, want "packets 2", got
table inet filter {
        counter iifgroupcount {
                packets 0 bytes 0
        }
}

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