test: Wait for NTP before using it
test: Wait for NTP before using it

After we start systemd-timedated service it may take a while until NTP
is ready. If we start it and immediately open dialog for changing time
NTP options may be disabled. If we wait until it `CanNTP` then things
are ready.

Closes #11443
mvollmer/cockpit,cockpituous/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,deryni/cockpit,moraleslazaro/cockpit,deryni/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,deryni/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,garrett/cockpit,garrett/cockpit,martinpitt/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,cockpituous/cockpit,cockpituous/cockpit,cockpit-project/cockpit,mvollmer/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit