selftests: mlxsw: Test vetoing of double sampling

Test that two sampling rules cannot be configured on the same port with
the same trigger.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>