selftests: mlxsw: rif_lag: Add a new selftest

This test verifies driver behavior with regards to creation of RIFs for a
LAG as ports are added or removed to/from it.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>