selftests: mlxsw: Add shared buffer configuration test

Test physical ports' shared buffer configuration options using random
values related to a specific configuration option. There are 3
configuration options: pool, TC bind and portpool.

Each sub-test, test a different configuration option and random the related
values as the follow:
 * For pools, pool's size will be randomized.
 * For TC bind, pool number and threshold will be randomized.
 * For portpools, threshold will be randomized.
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>