selftests: mlxsw: Add QOS test for maximum use of descriptors

Add an equivalent test to qos_burst, the test's purpose is same, but the
new test uses simpler topology and does not require forcing low speed.
In addition, it can be run Spectrum-2 and not only Spectrum-3+. The idea
is to use a shaper in order to limit the traffic and create congestion.

qos_burst test uses small pool, sends many small packets, and verify that
packets are not dropped, which means that many descriptors can be handled.
This test should check the change that commit c864769add96
("mlxsw: Configure descriptor buffers") pushed.

Instead, the new test tries to use more than 85% of maximum supported
descriptors. The idea is to use big pool (as much as the ASIC supports),
such that the pool size does not limit the traffic, then send many small
packets, which means that many descriptors are used, and check how many
packets the switch can handle.

The usage of shaper allows to run the test in all ASICs, regardless of
the CPU abilities, as it is able to create the congestion with low rate
of packets.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>