Add test for scaffold.0.dart and scaffold.2.dart (#148166)

Contributes to https://github.com/flutter/flutter/issues/130459

It adds test for
- `examples/api/lib/material/scaffold/scaffold.0.dart`
- `examples/api/lib/material/scaffold/scaffold.2.dart`

It also modifies the `scaffold.1_test.dart` because the only difference with scaffold 0 is the background color, so I figured the test should include it

dev/bots/check_code_samples.dart

@@ -351,9 +351,7 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/scaffold/scaffold.drawer.0_test.dart',
   'examples/api/test/material/scaffold/scaffold_messenger.of.0_test.dart',
   'examples/api/test/material/scaffold/scaffold_messenger.0_test.dart',
-  'examples/api/test/material/scaffold/scaffold.0_test.dart',
   'examples/api/test/material/scaffold/scaffold_state.show_bottom_sheet.0_test.dart',
-  'examples/api/test/material/scaffold/scaffold.2_test.dart',
   'examples/api/test/material/scaffold/scaffold_messenger_state.show_material_banner.0_test.dart',
   'examples/api/test/material/scaffold/scaffold_messenger.of.1_test.dart',
   'examples/api/test/material/scaffold/scaffold_messenger_state.show_snack_bar.0_test.dart',

examples/api/test/material/scaffold/scaffold.0_test.dart

@@ -0,0 +1,25 @@
+// 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 'package:flutter/material.dart';
+import 'package:flutter_api_samples/material/scaffold/scaffold.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('The count should be incremented when the floating action button is tapped', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ScaffoldExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'Sample Code'), findsOne);
+    expect(find.widgetWithIcon(FloatingActionButton, Icons.add), findsOne);
+    expect(find.text('You have pressed the button 0 times.'), findsOne);
+
+    for (int i = 1; i <= 5; i++) {
+      await tester.tap(find.byType(FloatingActionButton));
+      await tester.pump();
+      expect(find.text('You have pressed the button $i times.'), findsOne);
+    }
+  });
+}

examples/api/test/material/scaffold/scaffold.1_test.dart

@@ -21,5 +21,8 @@ void main() {
       await tester.pump();
       expect(find.text('You have pressed the button $i times.'), findsOne);
     }
+
+    final Scaffold scaffold = tester.firstWidget<Scaffold>(find.byType(Scaffold));
+    expect(scaffold.backgroundColor, Colors.blueGrey.shade200);
   });
 }

examples/api/test/material/scaffold/scaffold.2_test.dart

@@ -0,0 +1,27 @@
+// 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 'package:flutter/material.dart';
+import 'package:flutter_api_samples/material/scaffold/scaffold.2.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('The count should be incremented when the centered floating action button is tapped', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ScaffoldExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'Sample Code'), findsOne);
+    expect(find.widgetWithIcon(FloatingActionButton, Icons.add), findsOne);
+    expect(find.text('You have pressed the button 0 times.'), findsOne);
+    expect(find.byType(BottomAppBar), findsOne);
+    expect(tester.getCenter(find.byType(FloatingActionButton)).dx, 400);
+
+    for (int i = 1; i <= 5; i++) {
+      await tester.tap(find.byType(FloatingActionButton));
+      await tester.pump();
+      expect(find.text('You have pressed the button $i times.'), findsOne);
+    }
+  });
+}