node: fix arpping test
node: fix arpping test

In TestArpPingHandling, wait for all goroutines that are inserting the
new neighbors to finish before deleting the node.

Fixes: #16221
Suggested-by: Martynas Pumputis <6b0d31c0d563223024da45691584643ac78c96e8@lambda.lt>
Signed-off-by: Gilberto Bertin <e45dca12016e9fd4d68d2c7903502c5e1a1f4f1b@isovalent.com>

michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium