selftests: forwarding: Add resilient hashing test

Verify that IPv4 and IPv6 multipath forwarding works correctly with
resilient nexthop groups and with different weights.

Test that when the idle timer is not zero, the resilient groups are not
rebalanced - because the nexthop buckets are considered active - and the
initial weights (1:1) are used.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>