Added and refactored GroupObserver tests again
Added and refactored GroupObserver tests again

gjroelofs/Entitas-CSharp,npruehs/Entitas-CSharp,npruehs/Entitas-CSharp,gjroelofs/Entitas-CSharp,sschmid/Entitas-CSharp,vkuskov/Entitas-CSharp,vkuskov/Entitas-CSharp,robinryf/Entitas-CSharp,sschmid/Entitas-CSharp,robinryf/Entitas-CSharp