selftests: forwarding: Add multicast routing test

Introduce basic testing for both IPv4 and IPv6 multicast. The test creates
an (S,G) type route, sends traffic and verifies traffic arrives when the
route is present and then verifies traffic does not arrive after deleting
the route.
This test requires smcroute - https://github.com/troglobit/smcroute

 which
is a tool that allows creation of static multicast routes.

Signed-off-by: Nir Dotan <nird@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>