selftests: forwarding: router: Add test case for source IP in class E

Add test case to check that packets are not dropped when they need to be
routed and their source IP in class E, (i.e., 240.0.0.0 –
255.255.255.254).

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>