selftests: net: add new testcases for nexthop API compat mode sysctl

New tests to check route dump and notifications with
net.ipv4.nexthop_compat_mode on and off.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>