Currently, we manually populate neigh (ARP) entries for Cilium nodes (pkg/datapath/linux/node.go) to facilitate fib_lookup calls in the datapath.

However, we don't have any unit tests which would check the whole lifecycle of entries: adding a node -> doing arping -> inserting the entry -> removing the node -> removing the entry.

Ideally, we should not mock arping. Instead, we should create a netns, connect via a veth pair, assign IPs and then do the arping.