[go_router_builder] Update case sensitive test to go_router 16.0.0 (#9482)

Part of
- flutter/flutter#169809

Follow-up of 
- #9426
- #9444

Unskips and migrates the test about case sensitivity to the new version of `go_router` 15.2.4.

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: ValentinVignal

packages/go_router_builder/example/pubspec.yaml

@@ -9,7 +9,7 @@ dependencies:
   collection: ^1.15.0
   flutter:
     sdk: flutter
-  go_router: ^15.1.1
+  go_router: ^16.0.0
   provider: 6.0.5
 
 dev_dependencies:

packages/go_router_builder/example/test/case_sensitivity_example_test.dart

@@ -2,7 +2,6 @@
 // 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_test/flutter_test.dart';
 import 'package:go_router_builder_example/case_sensitive_example.dart';
@@ -27,28 +26,13 @@ void main() {
   });
 
   testWidgets(
-      // TODO(ValentinVignal): Migrate and unskip the test once
-      // https://github.com/flutter/packages/pull/9426 is merged
-      skip: true,
       'It should throw an error when the route is case sensitive and the path does not match',
       (WidgetTester tester) async {
-    final FlutterExceptionHandler? oldFlutterError = FlutterError.onError;
-    addTearDown(() => FlutterError.onError = oldFlutterError);
-    final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
-    FlutterError.onError = (FlutterErrorDetails details) {
-      errors.add(details);
-    };
-
     tester.platformDispatcher.defaultRouteNameTestValue = '/CASE-sensitive';
     await tester.pumpWidget(CaseSensitivityApp());
 
     expect(find.widgetWithText(AppBar, 'Case Sensitive'), findsNothing);
-    expect(errors, hasLength(1));
-    expect(
-      errors.single.exception,
-      isAssertionError,
-      reason: 'The path is case sensitive',
-    );
+    expect(find.text('Page Not Found'), findsOne);
   });
 
   testWidgets(