test: Fix check-machines race condition on ubuntu-stable
test: Fix check-machines race condition on ubuntu-stable

libvirtd.service asynchronously creates libvirt-sock, so the test needs
to wait until it exists before it can chown it.

Closes #13568
cockpit-project/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpit-project/cockpit,deryni/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,deryni/cockpit,garrett/cockpit,martinpitt/cockpit,martinpitt/cockpit,deryni/cockpit,garrett/cockpit,deryni/cockpit,martinpitt/cockpit,cockpituous/cockpit,cockpituous/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,cockpituous/cockpit,cockpit-project/cockpit,deryni/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpituous/cockpit,garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit