A zone is an isolated portion of an @xstate/test model. A zone defines the events it uses, the event that causes it to be entered and the states within the zone
The zones entryEvent is added as an event handler to the zones parent initial state. If this is a root zone passed to buildTestModel
this is the root state, if it's a sub zone (a state pointing at a zone), that is the initial state of the zone that state is contained in
Each state in a zone must contain a test property that validates that the state has been entered. At least one of the states requires it to be marked as the initial state of the zone with initial: true
An event handler in a zone state can only refer to events defined inside of the zone