selftests/net: replace obsolete NFT_CHAIN configuration

Replace old parameters with global NFT_NAT from commit db8ab388
("netfilter: nf_tables: merge ipv4 and ipv6 nat chain types")

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>