test: Wait for timesyncd restart in TestSystemInfo.testTimeServers
test: Wait for timesyncd restart in TestSystemInfo.testTimeServers

The dialog for applying NTP changes disables and re-enables NTP to let
timesyncd pick up the new settings. This triggers the ServerTime.changed
handler, so that the result of get_custom_ntp() changes while
change_systime_dialog.apply() runs.

To avoid that, let the test wait until applying a changed NTP setting
finished restarting timesyncd before changing the NTP config again. This
check is also desirable on its own right to ensure that settings will
actually become effective.

mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit