Add default value for `monitoringTestCirculationDelay`

[cliffparnitzky]

The code uses 10 as a default value. Set this in settings as default as well.

[cliffparnitzky]

This seems to be a good method: https://github.com/ContaoMonitoring/monitoring-client/blob/master/system/modules/MonitoringClient/dca/tl_settings.php#L90

[cliffparnitzky]

Implemented in fa0d581