selftests: mlxsw: qos_lib: Add a wrapper for running mlnx_qos

mlnx_qos is a script for configuration of DCB. Despite the name it is not
actually Mellanox-specific in any way. It is currently the only ad-hoc tool
available (in contrast to a daemon that manages an interface on an ongoing
basis). However, it is very verbose and parsing out error messages is not
really possible. Add a wrapper that makes it easier to use the tool.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>