selftests: mlxsw: Add resilient nexthop groups configuration tests

Test that unsupported resilient nexthop group configurations are
rejected and that offload / trap indication is correctly set on nexthop
buckets in a resilient group.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>