selftests: mlxsw: Add a test for UC behavior under MC flood

A so-called "MC-aware" mode has recently been enabled in mlxsw. In
MC-aware mode, BUM traffic is handled in a special way so that when a
switch is flooded with BUM, UC performance isn't unduly impacted.
Without enablement of this mode, a stream of BUM traffic can cause
sustained UC throughput drop in excess of 99 %.

Add a test for this behavior. Compare how much UC throughput degrades as
a stream of broadcast frames floods the switch. A minimal degradation is
tolerated to cover for glitches in traffic injection performance.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>