selftests: forwarding: Test IPv4 weighted nexthops

Use different weights for the multipath route configured on the first
router and check that the different flows generated by the first host
are distributed according to the provided weights.

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