selftests: Add version of router_multipath.sh using nexthop objects

Add a version of router_multipath.sh that uses nexthop objects for
routes.

Ido requested a version that does not cause regressions with mlxsw
testing since it does not support nexthop objects yet.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>