test: Fix flake in TestMachines.testLibvirt
test: Fix flake in TestMachines.testLibvirt

Recent libvirt versions ship socket units that auto-activate
libvirtd.service. This means that even when stopping libvirtd.service,
our page is still connected to libvirt and its requests cause the
service to auto-restart. Thus the empty state goes away all by itself
without having to press a button.

We should handle socket activation properly (issue #13563), but for now
just robustify that test case to really only test the service in
isolation.

Closes #13712

mvollmer/cockpit,cockpit-project/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,deryni/cockpit,deryni/cockpit,cockpituous/cockpit,martinpitt/cockpit,cockpituous/cockpit,deryni/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit,martinpitt/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,garrett/cockpit,garrett/cockpit,moraleslazaro/cockpit,deryni/cockpit,cockpituous/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpituous/cockpit,cockpit-project/cockpit,deryni/cockpit