test: Robustify and generalize libvirt zone setup in check-networking-firewall
test: Robustify and generalize libvirt zone setup in check-networking-firewall

check-machines-* can leave the default libvirt network stopped, so make
sure to start it.

Other tests need the libvirt zone as well, so move the code into
setUp().

Turn the positive OS list in testAddCustomServices into a negative list,
so that it is more robust when adding new OSes.

Closes #13720
garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpituous/cockpit,deryni/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit,garrett/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,cockpit-project/cockpit,cockpituous/cockpit,garrett/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,deryni/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,garrett/cockpit,martinpitt/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit