Write tests for important ECS use-cases

[sikanrong]

Need to update the test suite for the library such that we run through some key work-flows which represent:

• "Reactive" (change-event-driven) systems which trigger watchers for other Reactive systems
• "Non-Reactive" systems which update entities in an interval-based way
• Interactions between Reactive and Non-Reactive Systems.
• All sorts of different Watcher logic
• Different aspect logic in our Systems

Probably need to deal with even more subjects.