selftests: mlxsw: Add a test for soaking up a burst of traffic

Add a test that sends 1Gbps of traffic through the switch, into which it
then injects a burst of traffic and tests that there are no drops.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>