Write Tests for API Example of `form.0.dart` (#142635)

Write Tests for API Example of `form.0.dart`.

Part of #130459

dev/bots/check_code_samples.dart

@@ -406,7 +406,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/drag_target/draggable.0_test.dart',
   'examples/api/test/widgets/shared_app_data/shared_app_data.1_test.dart',
   'examples/api/test/widgets/shared_app_data/shared_app_data.0_test.dart',
-  'examples/api/test/widgets/form/form.0_test.dart',
   'examples/api/test/widgets/nested_scroll_view/nested_scroll_view_state.0_test.dart',
   'examples/api/test/widgets/nested_scroll_view/nested_scroll_view.2_test.dart',
   'examples/api/test/widgets/nested_scroll_view/nested_scroll_view.1_test.dart',

examples/api/test/widgets/form/form.0_test.dart

+// 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/widgets/form/form.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Form Smoke Test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.FormExampleApp(),
+    );
+    expect(find.widgetWithText(AppBar, 'Form Sample'), findsOneWidget);
+
+    final Finder textField = find.byType(TextField);
+    final Finder button = find.byType(ElevatedButton);
+    final TextField textFieldWidget = tester.widget<TextField>(textField);
+
+    expect(textField, findsOneWidget);
+    expect(button, findsOneWidget);
+
+    expect(textFieldWidget.controller?.text, '');
+    await tester.tap(button);
+    await tester.pumpAndSettle();
+    expect(find.text('Please enter some text'), findsOneWidget);
+
+    await tester.enterText(textField, 'Hello World');
+    expect(textFieldWidget.controller?.text, 'Hello World');
+    await tester.tap(button);
+    await tester.pumpAndSettle();
+    expect(find.text('Please enter some text'), findsNothing);
+  });
+}