Use a switch to turn this on or off - sometimes we need the system even if no entities run on it [anymore].
[x] Decide which entities to monitor in order to decide system exit, as a general rule.
[x] Change test scenarios to only do pings a limited number of times (e.g. 5 for simple scenarios, 10 for more complex), such that system exit is also tested.
Use a switch to turn this on or off - sometimes we need the system even if no entities run on it [anymore].