Add tests for navigator.0.dart (#150034)

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

It adds a test for
- `examples/api/lib/widgets/navigator/navigator.0.dart`

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

@@ -0,0 +1,35 @@
+// 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_api_samples/widgets/navigator/navigator.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('It should show the home page', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.NavigatorExampleApp(),
+    );
+
+    expect(find.text('Home Page'), findsOne);
+  });
+
+  testWidgets('It should start from the sign up page and follow the flow to reach the home page', (WidgetTester tester) async {
+    tester.platformDispatcher.defaultRouteNameTestValue = '/signup';
+    await tester.pumpWidget(
+      const example.NavigatorExampleApp(),
+    );
+
+    expect(find.text('Collect Personal Info Page'), findsOne);
+
+    await tester.tap(find.text('Collect Personal Info Page'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Choose Credentials Page'), findsOne);
+
+    await tester.tap(find.text('Choose Credentials Page'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Home Page'), findsOne);
+  });
+}