Add test for animated_positioned.0.dart API example. (#146720)

This PR contributes to https://github.com/flutter/flutter/issues/130459

### Description
- Adds `examples/api/test/widgets/implicit_animations/animated_positioned.0_test.dart` test

dev/bots/check_code_samples.dart

@@ -472,7 +472,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/focus_scope/focus_scope.0_test.dart',
   'examples/api/test/widgets/implicit_animations/animated_fractionally_sized_box.0_test.dart',
   'examples/api/test/widgets/implicit_animations/animated_align.0_test.dart',
-  'examples/api/test/widgets/implicit_animations/animated_positioned.0_test.dart',
   'examples/api/test/widgets/implicit_animations/sliver_animated_opacity.0_test.dart',
   'examples/api/test/widgets/scroll_view/custom_scroll_view.1_test.dart',
   'examples/api/test/widgets/inherited_notifier/inherited_notifier.0_test.dart',

examples/api/lib/widgets/implicit_animations/animated_positioned.0.dart

@@ -11,13 +11,19 @@ void main() => runApp(const AnimatedPositionedExampleApp());
 class AnimatedPositionedExampleApp extends StatelessWidget {
   const AnimatedPositionedExampleApp({super.key});
 
+  static const Duration duration = Duration(seconds: 2);
+  static const Curve curve = Curves.fastOutSlowIn;
+
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
       home: Scaffold(
         appBar: AppBar(title: const Text('AnimatedPositioned Sample')),
         body: const Center(
-          child: AnimatedPositionedExample(),
+          child: AnimatedPositionedExample(
+            duration: duration,
+            curve: curve,
+          ),
         ),
       ),
     );
@@ -25,10 +31,19 @@ class AnimatedPositionedExampleApp extends StatelessWidget {
 }
 
 class AnimatedPositionedExample extends StatefulWidget {
-  const AnimatedPositionedExample({super.key});
+  const AnimatedPositionedExample({
+    required this.duration,
+    required this.curve,
+    super.key,
+  });
+
+  final Duration duration;
+
+  final Curve curve;
 
   @override
-  State<AnimatedPositionedExample> createState() => _AnimatedPositionedExampleState();
+  State<AnimatedPositionedExample> createState() =>
+      _AnimatedPositionedExampleState();
 }
 
 class _AnimatedPositionedExampleState extends State<AnimatedPositionedExample> {
@@ -45,8 +60,8 @@ class _AnimatedPositionedExampleState extends State<AnimatedPositionedExample> {
             width: selected ? 200.0 : 50.0,
             height: selected ? 50.0 : 200.0,
             top: selected ? 50.0 : 150.0,
-            duration: const Duration(seconds: 2),
+            duration: widget.duration,
-            curve: Curves.fastOutSlowIn,
+            curve: widget.curve,
             child: GestureDetector(
               onTap: () {
                 setState(() {

examples/api/test/widgets/implicit_animations/animated_positioned.0_test.dart

@@ -0,0 +1,91 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/widgets/implicit_animations/animated_positioned.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets(
+    'AnimatedPositioned animates on tap',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.AnimatedPositionedExampleApp(),
+      );
+
+      final Finder positionedFinder = find.descendant(
+        of: find.byType(AnimatedPositioned),
+        matching: find.byType(Positioned),
+      );
+
+      const double beginWidth = 50.0;
+      const double endWidth = 200.0;
+      const double beginHeight = 200.0;
+      const double endHeight = 50.0;
+      const double beginTop = 150.0;
+      const double endTop = 50.0;
+
+      Positioned positioned = tester.widget(positionedFinder);
+      expect(positioned.width, beginWidth);
+      expect(positioned.height, beginHeight);
+      expect(positioned.top, beginTop);
+
+      // Tap on the 'Tap me' text to start the forward animation.
+      await tester.tap(find.text('Tap me'));
+      await tester.pump();
+
+      positioned = tester.widget(positionedFinder);
+      expect(positioned.width, beginWidth);
+      expect(positioned.height, beginHeight);
+      expect(positioned.top, beginTop);
+
+      // Advance animation to the middle.
+      await tester.pump(example.AnimatedPositionedExampleApp.duration ~/ 2);
+
+      final double t =
+          example.AnimatedPositionedExampleApp.curve.transform(0.5);
+
+      positioned = tester.widget(positionedFinder);
+      expect(positioned.width, lerpDouble(beginWidth, endWidth, t));
+      expect(positioned.height, lerpDouble(beginHeight, endHeight, t));
+      expect(positioned.top, lerpDouble(beginTop, endTop, t));
+
+      // Advance animation to the end.
+      await tester.pump(example.AnimatedPositionedExampleApp.duration ~/ 2);
+
+      positioned = tester.widget(positionedFinder);
+      expect(positioned.width, endWidth);
+      expect(positioned.height, endHeight);
+      expect(positioned.top, endTop);
+
+      // Tap on the 'Tap me' text again to start the reverse animation.
+      await tester.tap(find.text('Tap me'));
+      await tester.pump();
+
+      positioned = tester.widget(positionedFinder);
+      expect(positioned.width, endWidth);
+      expect(positioned.height, endHeight);
+      expect(positioned.top, endTop);
+
+      // Advance animation to the middle.
+      await tester.pump(example.AnimatedPositionedExampleApp.duration ~/ 2);
+
+      positioned = tester.widget(positionedFinder);
+      expect(positioned.width, lerpDouble(endWidth, beginWidth, t));
+      expect(positioned.height, lerpDouble(endHeight, beginHeight, t));
+      expect(positioned.top, lerpDouble(endTop, beginTop, t));
+
+      // Advance animation to the end.
+      await tester.pump(example.AnimatedPositionedExampleApp.duration ~/ 2);
+
+      positioned = tester.widget(positionedFinder);
+      expect(positioned.width, beginWidth);
+      expect(positioned.height, beginHeight);
+      expect(positioned.top, beginTop);
+    },
+  );
+}