test: Delete all unmanaged pods with a single kubectl delete
test: Delete all unmanaged pods with a single kubectl delete

In flake #14915, some pods are not found while attempting to restart
unmanaged pods on GKE. The assumption is that those pods are for some
reason deleted between the time we list unmanaged pods and the time we
delete them in RestartUnmanagedPodsInNamespace.

One reason this might happen is because we delete unmanaged pods one by
one. We first list all pods, then iterate on them, and every time a pod
is found unmanaged we run 'kubectl delete'. Since executing 'kubectl
delete' can take some time, it may take a while between when we take a
snapshot of all pods and when we delete the last pods in the list.

To avoid this, we can list all pods and iterate on them to find unmanaged
pods. Then, in a single go we can delete all unmanaged pods with a
single 'kubectl delete' command. This commit implements that approach.

Signed-off-by: Paul Chaignon <a027184a55211cd23e3f3094f1fdc728df5e0500@cilium.io>

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