Fix cleaning up port-forwards in docs test

[su225]

There are many tests where we use killall in documentation, but we use pgrep istioctl | xargs kill. One reason is that killall is not present in the istio-testing/build-tools container. Another strange thing is we have to use these commands both in test as well as cleanup phase which is weird. This must be fixed to use the snippet from the documentation page for more accuracy.

#8003 (comment)

Some examples

1. https://preliminary.istio.io/latest/docs/tasks/observability/distributed-tracing/zipkin/
2. https://preliminary.istio.io/latest/docs/tasks/observability/distributed-tracing/jaeger/
3. https://preliminary.istio.io/latest/docs/tasks/observability/kiali/
4. https://preliminary.istio.io/latest/docs/tasks/observability/metrics/querying-metrics/
5. https://preliminary.istio.io/latest/docs/tasks/observability/metrics/using-istio-dashboard/
6. https://preliminary.istio.io/latest/docs/tasks/observability/metrics/tcp-metrics/