Add test for sliver_animated_opacity.0.dart API example. (#146722)

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

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

dev/bots/check_code_samples.dart

@@ -470,7 +470,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/focus_scope/focus.1_test.dart',
   '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/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/test/animation/curves/curve2_d.0_test.dart',

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

@@ -11,13 +11,19 @@ void main() => runApp(const SliverAnimatedOpacityExampleApp());
 class SliverAnimatedOpacityExampleApp extends StatelessWidget {
   const SliverAnimatedOpacityExampleApp({super.key});
 
+  static const Duration duration = Duration(milliseconds: 500);
+  static const Curve curve = Curves.easeInOut;
+
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
       home: Scaffold(
         appBar: AppBar(title: const Text('SliverAnimatedOpacity Sample')),
         body: const Center(
-          child: SliverAnimatedOpacityExample(),
+          child: SliverAnimatedOpacityExample(
+            duration: duration,
+            curve: curve,
+          ),
         ),
       ),
     );
@@ -25,13 +31,23 @@ class SliverAnimatedOpacityExampleApp extends StatelessWidget {
 }
 
 class SliverAnimatedOpacityExample extends StatefulWidget {
-  const SliverAnimatedOpacityExample({super.key});
+  const SliverAnimatedOpacityExample({
+    required this.duration,
+    required this.curve,
+    super.key,
+  });
+
+  final Duration duration;
+
+  final Curve curve;
 
   @override
-  State<SliverAnimatedOpacityExample> createState() => _SliverAnimatedOpacityExampleState();
+  State<SliverAnimatedOpacityExample> createState() =>
+      _SliverAnimatedOpacityExampleState();
 }
 
-class _SliverAnimatedOpacityExampleState extends State<SliverAnimatedOpacityExample>
+class _SliverAnimatedOpacityExampleState
+    extends State<SliverAnimatedOpacityExample>
     with SingleTickerProviderStateMixin {
   bool _visible = true;
 
@@ -40,7 +56,8 @@ class _SliverAnimatedOpacityExampleState extends State<SliverAnimatedOpacityExam
     return CustomScrollView(slivers: <Widget>[
       SliverAnimatedOpacity(
         opacity: _visible ? 1.0 : 0.0,
-        duration: const Duration(milliseconds: 500),
+        duration: widget.duration,
+        curve: widget.curve,
         sliver: SliverFixedExtentList(
           itemExtent: 100.0,
           delegate: SliverChildBuilderDelegate(

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

@@ -0,0 +1,84 @@
+// 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/sliver_animated_opacity.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets(
+    'SilverAnimatedOpacity animates on FloatingActionButton tap',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.SliverAnimatedOpacityExampleApp(),
+      );
+
+      final Finder fadeTransitionFinder = find.descendant(
+        of: find.byType(SliverAnimatedOpacity),
+        matching: find.byType(SliverFadeTransition),
+      );
+
+      const double beginOpacity = 1.0;
+      const double endOpacity = 0.0;
+
+      SliverFadeTransition fadeTransition = tester.widget(fadeTransitionFinder);
+      expect(fadeTransition.opacity.value, beginOpacity);
+
+      // Tap on the FloatingActionButton to start the forward animation.
+      await tester.tap(find.byType(FloatingActionButton));
+      await tester.pump();
+
+      fadeTransition = tester.widget(fadeTransitionFinder);
+      expect(fadeTransition.opacity.value, beginOpacity);
+
+      // Advance animation to the middle.
+      await tester.pump(example.SliverAnimatedOpacityExampleApp.duration ~/ 2);
+
+      fadeTransition = tester.widget(fadeTransitionFinder);
+      expect(
+        fadeTransition.opacity.value,
+        lerpDouble(
+          beginOpacity,
+          endOpacity,
+          example.SliverAnimatedOpacityExampleApp.curve.transform(0.5),
+        ),
+      );
+
+      // Advance animation to the end.
+      await tester.pump(example.SliverAnimatedOpacityExampleApp.duration ~/ 2);
+
+      fadeTransition = tester.widget(fadeTransitionFinder);
+      expect(fadeTransition.opacity.value, endOpacity);
+
+      // Tap on the FloatingActionButton again to start the reverse animation.
+      await tester.tap(find.byType(FloatingActionButton));
+      await tester.pump();
+
+      fadeTransition = tester.widget(fadeTransitionFinder);
+      expect(fadeTransition.opacity.value, endOpacity);
+
+      // Advance animation to the middle.
+      await tester.pump(example.SliverAnimatedOpacityExampleApp.duration ~/ 2);
+
+      fadeTransition = tester.widget(fadeTransitionFinder);
+      expect(
+        fadeTransition.opacity.value,
+        lerpDouble(
+          endOpacity,
+          beginOpacity,
+          example.SliverAnimatedOpacityExampleApp.curve.transform(0.5),
+        ),
+      );
+
+      // Advance animation to the end.
+      await tester.pump(example.SliverAnimatedOpacityExampleApp.duration ~/ 2);
+
+      fadeTransition = tester.widget(fadeTransitionFinder);
+      expect(fadeTransition.opacity.value, beginOpacity);
+    },
+  );
+}