selftests: netfilter: add ipvs nat test case

Test virtual server via NAT.

Tested:
# selftests: netfilter: ipvs.sh
# Testing DR mode...
# Testing NAT mode...
# ipvs.sh: PASS

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: Simon Horman <horms@verge.net.au>