iptables_tests.sh: nat table can have 4 chains
It is considered by iptables_tests.sh a failure When nat table chain count is
not 3, while recent kernels can have 4 built-in chains in nat table, namely
PREROUTING, OUTPUT, POSTROUTING and INPUT.
Signed-off-by: Simon Xu <email@example.com>
Reviewed-by: Wanlong Gao <firstname.lastname@example.org>