selftests: forwarding: Introduce basic shared blocks tests

Test shared block infrastructure. This is a basic test that shares TC
block in between 2 clsact qdiscs.

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