selftests: mlxsw: Test policers' occupancy

Test that policers shared by different tc filters are correctly
reference counted by observing policers' occupancy via devlink-resource.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>