selftests: forwarding: Add reverse path forwarding (RPF) test cases

In case a packet is routed using a multicast route whose specified
ingress interface does not match the interface from which the packet was
received, the packet is dropped.

Add IPv4 and IPv6 test cases for above mentioned scenario.

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