selftests: forwarding: add basic QoS classification test for Ocelot switches

Test basic (port-default, VLAN PCP and IP DSCP) QoS classification for
Ocelot switches. Advanced QoS classification using tc filters is covered
by tc_flower_chains.sh in the same directory.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220502155424.4098917-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>