selftests: mlxsw: Test RIF MAC vetoing

Test that attempts to change address in a way that violates Spectrum
requirements are vetoed with extack.

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