selftests: nft_flowtable.sh: check ingress/egress chain too

Make sure flowtable interacts correctly with ingress and egress
chains, i.e. those get handled before and after flow table respectively.

Adds three more tests:
1. repeat flowtable test, but with 'ip dscp set cs3' done in
   inet forward chain.

Expect that some packets have been mangled (before flowtable offload
became effective) while some pass without mangling (after offload
succeeds).

2. repeat flowtable test, but with 'ip dscp set cs3' done in
   veth0:ingress.

Expect that all packets pass with cs3 dscp field.

3. same as 2, but use veth1:egress.  Expect the same outcome.

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