selftests: mlxsw: Add a new test for strict priority

Test that when strict priority is configured on a system, the
higher-priority traffic does actually win all the available bandwidth.
The test uses a similar approach to qos_mc_aware.sh to run and account
the traffic.

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>