selftests: netdevsim: Add test for resilient nexthop groups offload API

Test various aspects of the resilient nexthop group offload API on top
of the netdevsim implementation. Both good and bad flows are tested.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Co-developed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>