selftests: forwarding: Add multipath test for L4 hashing

Add IPv6 multipath test using L4 hashing. Created with inputs from
Ido Schimmel.

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