refactor: Fix unrelated types reported by DCM (#3023)

DCM recently started reporting
`avoid-collection-methods-with-unrelated-types` for the quadtree class,
this PR fixes the reported issues.

packages/flame/lib/src/collisions/broadphase/quadtree/quadtree.dart

@@ -190,11 +190,11 @@ class QuadTree<T extends Hitbox<T>> {
   }
 
   void remove(T hitbox, {bool keepOldPosition = false}) {
-    final node = _hitboxAtNode.remove(hitbox);
+    final node = _hitboxAtNode.remove(hitbox as ShapeHitbox);
     if (node != null) {
       node.hitboxes.remove(hitbox);
       if (!keepOldPosition) {
-        _oldPositionByItem.remove(hitbox);
+        _oldPositionByItem.remove(hitbox as ShapeHitbox);
       }
     }
   }
@@ -273,7 +273,7 @@ class QuadTree<T extends Hitbox<T>> {
   }
 
   bool hasMoved(T hitbox) {
-    final lastPos = _oldPositionByItem[hitbox];
+    final lastPos = _oldPositionByItem[hitbox as ShapeHitbox];
     if (lastPos == null) {
       return true;
     }

packages/flame/test/collisions/collision_test_helpers.dart

@@ -126,7 +126,7 @@ class TestBlock extends PositionComponent with CollisionCallbacks {
     return activeCollisions.contains(other);
   }
 
-  bool collidedWithExactly(List<CollisionCallbacks> collidables) {
+  bool collidedWithExactly(List<PositionComponent> collidables) {
     final otherCollidables = collidables.toSet()..remove(this);
     return activeCollisions.containsAll(otherCollidables) &&
         otherCollidables.containsAll(activeCollisions);