selftests: forwarding: Add $h3's clsact to mirror_topo_lib.sh

Having a clsact qdisc on $h3 is useful in several tests, and will be
useful in more tests to come. Move the registration from all the tests
that need it into the topology file itself.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>