Add test for segmented_button.0.dart (#150676)

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

It adds a test for
- `examples/api/test/material/segmented_button/segmented_button.0_test.dart`

dev/bots/check_code_samples.dart

@@ -336,7 +336,6 @@ final Set<String> _knownMissingTests = <String>{
   '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/segmented_button/segmented_button.0_test.dart',
   'examples/api/test/material/app_bar/sliver_app_bar.2_test.dart',
   'examples/api/test/material/app_bar/sliver_app_bar.3_test.dart',
   'examples/api/test/material/navigation_rail/navigation_rail.extended_animation.0_test.dart',

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

@@ -0,0 +1,110 @@
+// 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/foundation.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/material/segmented_button/segmented_button.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Segmented button can be used with a single selection', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.SegmentedButtonApp(),
+    );
+
+    void expectOneCalendarButton(example.Calendar period) {
+      expect(
+        find.byWidgetPredicate(
+          (Widget widget) => widget is SegmentedButton<example.Calendar> && setEquals(widget.selected, <example.Calendar>{period}),
+        ),
+        findsOne,
+      );
+    }
+
+    expect(find.text('Single choice'), findsOne);
+    expect(find.text('Day'), findsOne);
+    expect(find.text('Week'), findsOne);
+    expect(find.text('Month'), findsOne);
+    expect(find.text('Year'), findsOne);
+
+    expectOneCalendarButton(example.Calendar.day);
+
+    // Select the day.
+    await tester.tap(find.text('Week'));
+    await tester.pump();
+
+    expectOneCalendarButton(example.Calendar.week);
+
+    // Select the month.
+    await tester.tap(find.text('Month'));
+    await tester.pump();
+
+    expectOneCalendarButton(example.Calendar.month);
+
+    // Select the year.
+    await tester.tap(find.text('Year'));
+    await tester.pump();
+
+    expectOneCalendarButton(example.Calendar.year);
+
+    // Select the day.
+    await tester.tap(find.text('Day'));
+    await tester.pump();
+
+    expectOneCalendarButton(example.Calendar.day);
+
+    // Try to unselect the day.
+    await tester.tap(find.text('Day'));
+    await tester.pump();
+
+    expectOneCalendarButton(example.Calendar.day);
+  });
+
+  testWidgets('Segmented button can be used with a multiple selection', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.SegmentedButtonApp(),
+    );
+
+    void expectSizeButtons(Set<example.Sizes> sizes) {
+      expect(
+        find.byWidgetPredicate(
+          (Widget widget) => widget is SegmentedButton<example.Sizes> && setEquals(widget.selected, sizes),
+        ),
+        findsOne,
+      );
+    }
+
+    expect(find.text('Multiple choice'), findsOne);
+    expect(find.text('XS'), findsOne);
+    expect(find.text('S'), findsOne);
+    expect(find.text('M'), findsOne);
+    expect(find.text('L'), findsOne);
+    expect(find.text('XL'), findsOne);
+
+    expectSizeButtons(const <example.Sizes>{example.Sizes.large, example.Sizes.extraLarge});
+
+    // Select everything.
+    await tester.tap(find.text('XS'));
+    await tester.pump();
+    await tester.tap(find.text('S'));
+    await tester.pump();
+    await tester.tap(find.text('M'));
+    await tester.pump();
+
+    expectSizeButtons(example.Sizes.values.toSet());
+
+    // Unselect everything but XS.
+    await tester.tap(find.text('S'));
+    await tester.pump();
+    await tester.tap(find.text('M'));
+    await tester.pump();
+    await tester.tap(find.text('L'));
+    await tester.pump();
+    await tester.tap(find.text('XL'));
+    await tester.pump();
+
+    expectSizeButtons(const <example.Sizes>{example.Sizes.extraSmall});
+  });
+}