selftests: forwarding: Create test topology for multipath routing

Create a topology with two hosts, each directly connected to a different
router. Both routers are connected using two links, enabling multipath
routing.

Test IPv4 and IPv6 ping using default MTU and large MTU.

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