selftests: netfilter: add IPV6_TUNNEL to config

selftests: netfilter: add IPV6_TUNNEL to config

The script now requires IPV6 tunnel support, enable this.
This should have caught by CI, but as the config option is missing,
the tunnel interface isn't added.  This results in an error cascade
that ends with "route change default" failure.

That in turn means the "ipv6 tunnel" test re-uses the previous
test setup so the "ip6ip6" test passes and script returns 0.

Make sure to catch such bugs, set ret=1 if device cannot be added
and delete the old default route before installing the new one.

After this change, IPV6_TUNNEL=n kernel builds fail with the expected
  FAIL: flow offload for ns1/ns2 with IP6IP6 tunnel

... while builds with IPV6_TUNNEL=m pass as before.

Fixes: 5e5180352193 ("selftests: netfilter: nft_flowtable.sh: Add IP6IP6 flowtable selftest")
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Florian Westphal <fw@strlen.de>