selftests: mlxsw: Add test for trust-DSCP

Add a test that exercises the new code. Send DSCP-tagged packets, and
observe how they are prioritized in the switch and the DSCP is updated
on egress again.

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>