[UnitTest] Check for specific amount of events to arrive

This issue is based on this discussion.

I want to create a test that checks for a specific number of events being published. The usecase here is that it should only publish events until a given limit of currently executing jobs is reached. This limit is used to prevent one instance of the service to block all jobs while another instance is idle.

scenario.stimulate(() -> collector.poll())
            .andWaitAtMost(Duration.ofSeconds(5))
            .andWaitForEventOfType(NewJobEvent.class)
           .count(3)
            .toArrive();

Start by tracing the scenario.stimulate(...).andWaitForEventOfType(...).count(3).toArrive() API shown in the issue and review discussion 589 for the intended behavior. Done when a test can assert that exactly three NewJobEvent events arrive within the existing wait flow.