Add test for expansion_panel_list.expansion_panel_list_radio.0_test.dart (flutter#151730)

Contributes to flutter#130459

It adds a test for
- `examples/api/test/material/expansion_panel/expansion_panel_list.expansion_panel_list_radio.0_test.dart`

Author: ValentinVignal

dev/bots/check_code_samples.dart

@@ -321,7 +321,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/platform_menu_bar/platform_menu_bar.0_test.dart',
   'examples/api/test/material/flexible_space_bar/flexible_space_bar.0_test.dart',
   'examples/api/test/material/chip/deletable_chip_attributes.on_deleted.0_test.dart',
-  'examples/api/test/material/expansion_panel/expansion_panel_list.expansion_panel_list_radio.0_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.0_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.1_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.2_test.dart',

examples/api/test/material/expansion_panel/expansion_panel_list.expansion_panel_list_radio.0_test.dart

@@ -0,0 +1,44 @@
+// 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/expansion_panel/expansion_panel_list.expansion_panel_list_radio.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('ExpansionPanelList.radio can expand one item at the time', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ExpansionPanelListRadioExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'ExpansionPanelList.radio Sample'), findsOne);
+    expect(find.byType(ExpansionPanelList), findsOne);
+    for (int i = 0; i < 8; i++) {
+      expect(find.widgetWithText(ListTile, 'Panel $i'), findsOne);
+    }
+
+    // The default expanded item is 2.
+    for (int i = 0; i < 8; i++) {
+      expect(
+        tester.widget<ExpandIcon>(find.byType(ExpandIcon).at(i)).isExpanded,
+        i == 2,
+        reason: 'Only the panel 2 should be expanded',
+      );
+    }
+
+    // Open all the panels one by one.
+    for (int index = 0; index < 8; index++) {
+      await tester.ensureVisible(find.byType(ExpandIcon).at(index));
+      await tester.tap(find.byType(ExpandIcon).at(index));
+      await tester.pumpAndSettle();
+      for (int i = 0; i < 8; i++) {
+        expect(
+          tester.widget<ExpandIcon>(find.byType(ExpandIcon).at(i)).isExpanded,
+          i == index,
+          reason: 'Only the panel $index should be expanded',
+        );
+      }
+    }
+  });
+}