selftests: mlxsw: Add nexthop objects configuration tests

Test that unsupported nexthop objects are rejected and that offload
indication is correctly set on: nexthop objects, nexthop group objects
and routes associated these objects.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>