test: get cilium pods inside background closure
test: get cilium pods inside background closure

BackgroundReport dumps the result of the given command (e.g., uptime)
executed on the cilium pods every 5 seconds. The name of the cilium pods
are retrieved once at the beginning. This has the problem that if the
names of the cilium pods change, spurious messages like the following
will appear on the logs:

```
time="2020-11-17T00:15:08Z" level=error msg="Error executing command 'kubectl exec -n kube-system cilium-nr2hm -- uptime'" error="exit status 1"
cmd: "kubectl exec -n kube-system cilium-nr2hm -- uptime" exitCode: 1 duration: 64.011551ms stdout:

err:
exit status 1
stderr:
Error from server (NotFound): pods "cilium-nr2hm" not found
```

Fix this by retrieving the names of the pods within the closure, rather
than only at the beginning.

Signed-off-by: Kornilios Kourtis <fbd1fdd48e316a736711a3b30106195507321a09@isovalent.com>

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