Add tests for editable_text.on_changed.0.dart API example. (#148874)

This PR contributes to flutter/flutter#130459

### Description
- Adds tests for `examples/api/lib/widgets/editable_text/editable_text.on_changed.0.dart`

Author: ksokolovskyi

dev/bots/check_code_samples.dart

@@ -395,7 +395,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/interactive_viewer/interactive_viewer.transformation_controller.0_test.dart',
   'examples/api/test/widgets/interactive_viewer/interactive_viewer.0_test.dart',
   'examples/api/test/widgets/notification_listener/notification.0_test.dart',
-  'examples/api/test/widgets/editable_text/editable_text.on_changed.0_test.dart',
   'examples/api/test/widgets/overscroll_indicator/glowing_overscroll_indicator.1_test.dart',
   'examples/api/test/widgets/overscroll_indicator/glowing_overscroll_indicator.0_test.dart',
   'examples/api/test/widgets/tween_animation_builder/tween_animation_builder.0_test.dart',

examples/api/test/widgets/editable_text/editable_text.on_changed.0_test.dart

@@ -0,0 +1,68 @@
+// 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/editable_text/editable_text.on_changed.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Verify correct labels are displayed', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.OnChangedExampleApp(),
+    );
+
+    expect(
+      find.text('What number comes next in the sequence?'),
+      findsOneWidget,
+    );
+    expect(find.text('1, 1, 2, 3, 5, 8...?'), findsOneWidget);
+  });
+
+  testWidgets('Does not show dialog when answer is not correct', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.OnChangedExampleApp(),
+    );
+
+    await tester.enterText(find.byType(TextField), '33');
+    await tester.pumpAndSettle();
+
+    expect(find.byType(AlertDialog), findsNothing);
+  });
+
+  testWidgets('Shows dialog when answer is correct', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.OnChangedExampleApp(),
+    );
+
+    await tester.enterText(find.byType(TextField), '13');
+    await tester.pumpAndSettle();
+
+    expect(find.byType(AlertDialog), findsOneWidget);
+    expect(find.text('That is correct!'), findsOneWidget);
+    expect(find.text('13 is the right answer.'), findsOneWidget);
+    expect(find.text('OK'), findsOneWidget);
+  });
+
+  testWidgets('Closes dialog on OK button tap', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.OnChangedExampleApp(),
+    );
+
+    await tester.enterText(find.byType(TextField), '13');
+    await tester.pumpAndSettle();
+
+    expect(find.byType(AlertDialog), findsOneWidget);
+
+    await tester.tap(
+      find.ancestor(
+        of: find.text('OK'),
+        matching: find.byType(TextButton),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.byType(AlertDialog), findsNothing);
+  });
+}