selftests: add route_localnet test script

Add a simple scripts to exercise several situations when enable
route_localnet.

Signed-off-by: Shijie Luo <luoshijie1@huawei.com>
Signed-off-by: Zhiqiang liu <liuzhiqiang26@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>