Add tests for sliver grid delegate with fixed cross axis count examples (flutter#153500)

Contributes to flutter#130459

It adds a test for
- `examples/api/lib/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.0.dart`
- `examples/api/lib/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.1.dart`

I also fixed a mistake in the documentation

Author: ValentinVignal

dev/bots/check_code_samples.dart

@@ -322,8 +322,6 @@ final Set<String> _knownMissingTests = <String>{
   '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/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.0_test.dart',
-  'examples/api/test/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.1_test.dart',
   'examples/api/test/painting/star_border/star_border.0_test.dart',
   'examples/api/test/widgets/navigator/navigator.restorable_push_and_remove_until.0_test.dart',
   'examples/api/test/widgets/navigator/navigator.restorable_push.0_test.dart',

examples/api/lib/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.0.dart

@@ -30,7 +30,7 @@ class SliverGridDelegateWithFixedCrossAxisCountExample extends StatelessWidget {
     return GridView(
       gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(
         crossAxisCount: 4,
-        childAspectRatio: 0.5,
+        childAspectRatio: 2,
       ),
       children: List<Widget>.generate(20, (int i) {
         return Builder(builder: (BuildContext context) {

examples/api/test/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.0_test.dart

@@ -0,0 +1,22 @@
+// 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/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Each tiles should have a width of 200.0 and a height of 100.0', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.SliverGridDelegateWithFixedCrossAxisCountExampleApp(),
+    );
+
+    for (int i = 0; i < 20; i++) {
+      expect(find.text('$i'), findsOne);
+      final Element element = tester.element(find.text('$i'));
+
+      expect(element.size, const Size(200, 100));
+    }
+  });
+}

examples/api/test/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.1_test.dart

@@ -0,0 +1,22 @@
+// 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/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.1.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Each tiles should have a width of 200.0 and a height of 150.0', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.SliverGridDelegateWithFixedCrossAxisCountExampleApp(),
+    );
+
+    for (int i = 0; i < 4; i++) {
+      expect(find.text('$i'), findsOne);
+      final Element element = tester.element(find.text('$i'));
+
+      expect(element.size, const Size(200, 150));
+    }
+  });
+}