refactor: used variables for fixtures

lib/game/components/flipper.dart

@@ -167,7 +167,8 @@ class Flipper extends PositionBodyComponent with KeyboardHandler {
           : (size.x / 2) - bigCircleShape.radius,
       0,
     );
-    fixtures.add(FixtureDef(bigCircleShape));
+    final bigCircleFixtureDef = FixtureDef(bigCircleShape);
+    fixtures.add(bigCircleFixtureDef);
 
     final smallCircleShape = CircleShape()..radius = bigCircleShape.radius / 2;
     smallCircleShape.position.setValues(
@@ -176,7 +177,8 @@ class Flipper extends PositionBodyComponent with KeyboardHandler {
           : -(size.x / 2) + smallCircleShape.radius,
       0,
     );
-    fixtures.add(FixtureDef(smallCircleShape));
+    final smallCircleFixtureDef = FixtureDef(smallCircleShape);
+    fixtures.add(smallCircleFixtureDef);
 
     final trapeziumVertices = isLeft
         ? [

test/game/components/ball_test.dart

@@ -19,6 +19,7 @@ void main() {
       'loads correctly',
       (game) async {
         final ball = Ball(position: Vector2.zero());
+        await game.ready();
         await game.ensureAdd(ball);
 
         expect(game.contains(ball), isTrue);