selftests: mlxsw: Add shared buffer traffic test

Test the max shared buffer occupancy for port's pool and port's TC's (using
different types of packets).

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>