selftests: net: add missing config for nftables-backed iptables

[ Upstream commit 59c93583

 ]

Modern OSes use iptables implementation with nf_tables as a backend,
e.g.:

$ iptables -V
iptables v1.8.8 (nf_tables)

Pablo points out that we need CONFIG_NFT_COMPAT to make that work,
otherwise we see a lot of:

  Warning: Extension DNAT revision 0 not supported, missing kernel module?

with DNAT being just an example here, other modules we need
include udp, TTL, length etc.

Link: https://lore.kernel.org/r/20240126201308.2903602-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: f7c25d8e

 ("selftests: net: add missing config for pmtu.sh tests")
Signed-off-by: Sasha Levin <sashal@kernel.org>