selftests: forwarding: Test IPv6 weighted nexthops

Have one host generate 16K IPv6 echo requests with a random flow label
and check that they are distributed between both multipath links
according to the provided weights.

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