test: Quarantine K8sIstioTest while flaky
test: Quarantine K8sIstioTest while flaky

This commit quarantines the Istio test on K8s 1.19 as it is currently
flaky and makes subsequent tests fail.

Related: #12993
Signed-off-by: Paul Chaignon <a027184a55211cd23e3f3094f1fdc728df5e0500@cilium.io>

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