test: Robustify NM online hack for Ubuntu
test: Robustify NM online hack for Ubuntu

Use a dummy interface instead of relying on libvirt's default network
virbr0. This decouples this hack from the Machines tests, drops the
dependency on virbr0 being up, and stops multilating that.

Closes #13672

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