selftests: forwarding: Add blackhole nexthops tests

Test that IPv4 and IPv6 ping fail when the route is using a blackhole
nexthop or a group with a blackhole nexthop. Test that ping passes when
the route starts using a valid nexthop.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>