selftests: mlxsw: Add qos_lib.sh

Extract reusable code from qos_mc_aware.sh and put into a new library.

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>