Test SetUp for ECS

ArenaTestFixtures.cs

public class ArenaTestFixtures {

    protected World previousWorld;
    protected World currentWorld;
    protected EntityManager entityManager;

    public RetrievalSystem RetrievalSystem {
        get => World.Active.GetOrCreateManager<RetrievalSystem>();
    }

    [SetUp]
    public virtual void SetUp() {
        previousWorld = World.Active;
        currentWorld = World.Active = new World("Test World");

        entityManager = currentWorld.GetOrCreateManager<EntityManager>();
    }

    [TearDown]
    public virtual void TearDown() {
        if (entityManager != null) {
            currentWorld.Dispose();
            currentWorld = null;

            World.Active = previousWorld;
            previousWorld = null;
            entityManager = null;
        }
    }
}