selftests: netfilter: add bridge tunnel flowtable regression

selftests: netfilter: add bridge tunnel flowtable regression

Add a nft_flowtable.sh regression test for the bridge direct-xmit plus
IPIP/IP6IP6 underlay configuration that reproduces the reachable
DIRECT+tunnel tuple combination exercised by the flowtable fix.

The test reuses the existing bridge and tunnel topology, installs flow
rules for the tunnel egress and bridge reply path, verifies IPv4 and
IPv6 forwarding, and checks the flowtable counters after the transfer.

Signed-off-by: Zhengyang Chen <chzhengyang2023@lzu.edu.cn>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Florian Westphal <fw@strlen.de>