selftests: forwarding: router: Add test case for source IP equals destination IP

Add test case to check that packets are not dropped when they need to be
routed and their source IP equals to their destination IP.

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