selftests: netfilter: add flowtable test script

Exercises 3 cases:

1. no pmtu discovery (need to frag)
2. no PMTUd + NAT (don't flag packets as invalid from conntrack)
3. PMTU + NAT (need to send icmp error)

The first two cases make sure we handle fragments correctly, i.e.
pass them to classic forwarding path.

Third case checks we offload everything (in the test case,
PMTUd will kick in so all packets should be within link mtu).

Nftables rules will filter packets that are supposed to be
handled by the fast-path.

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