test: Wait for non-disabled button in TestFirewall.testAddServices
test: Wait for non-disabled button in TestFirewall.testAddServices

When removing a service, the button is initially disabled, until reading
the zones is complete.

Closes #11820
martinpitt/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,deryni/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,deryni/cockpit,garrett/cockpit,garrett/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,deryni/cockpit,garrett/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,deryni/cockpit,garrett/cockpit,cockpituous/cockpit,deryni/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpituous/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit