Add generic PositionComponent test helper

test/effects/effect_test_utils.dart

@@ -0,0 +1,63 @@
+import 'dart:math';
+
+import 'package:flame/anchor.dart';
+import 'package:flame/components/position_component.dart';
+import 'package:flame/effects/effects.dart';
+import 'package:flame/extensions/vector2.dart';
+import 'package:flame/game/base_game.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+final Random random = Random();
+
+class Callback {
+  bool isCalled = false;
+  void call() => isCalled = true;
+}
+
+void effectTest(
+    WidgetTester tester,
+    PositionComponent component,
+    PositionComponentEffect effect, {
+      bool hasFinished = true,
+      bool hitEdges = false,
+      double iterations = 1.0,
+      double expectedAngle = 0.0,
+      Vector2 expectedPosition,
+      Vector2 expectedSize,
+    }) async {
+  expectedPosition ??= Vector2.zero();
+  expectedSize ??= Vector2.all(100.0);
+  final Callback callback = Callback();
+  effect.onComplete = callback.call;
+  final BaseGame game = BaseGame();
+  final double duration = (random.nextDouble() * 100).roundToDouble();
+  game.add(component);
+  component.addEffect(effect);
+  await tester.pumpWidget(game.widget);
+  double timeLeft = iterations * duration;
+  while(timeLeft > 0) {
+    final double stepDelta = hitEdges ? 0.01 : random.nextDouble() / 10;
+    game.update(stepDelta);
+    timeLeft -= stepDelta;
+  }
+  expect(effect.hasFinished(), hasFinished);
+  expect(callback.isCalled, hasFinished);
+  expect(component.angle, expectedAngle);
+  expect(component.position, expectedPosition);
+  expect(component.size, expectedSize);
+  expect(component.effects.isEmpty, hasFinished);
+}
+
+class Square extends PositionComponent {
+  Square({
+    Vector2 position,
+    Vector2 size,
+    double angle,
+    Anchor anchor,
+  }) {
+    this.position = position ?? Vector2.zero();
+    this.size = size ?? Vector2.all(100.0);
+    this.angle = angle ?? 0.0;
+    this.anchor = anchor ?? Anchor.center;
+  }
+}