test(cupertino_http): Retry cancellation tests.

pkgs/cupertino_http/example/integration_test/url_session_task_test.dart

@@ -2,6 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:async';
 import 'dart:io';
 
 import 'package:cupertino_http/cupertino_http.dart';
@@ -207,18 +208,23 @@ void testURLSessionTaskCommon(
   group('task states', () {
     late HttpServer server;
     late URLSessionTask task;
+    late Completer<void> requestCompleter;
+
     setUp(() async {
+      requestCompleter = Completer<void>();
       server = (await HttpServer.bind('localhost', 0))
         ..listen((request) async {
           await request.drain<void>();
           request.response.headers.set('Content-Type', 'text/plain');
           request.response.write('Hello World');
+          await requestCompleter.future;
           await request.response.close();
         });
       final session = URLSession.sharedSession();
       task = f(session, Uri.parse('http://localhost:${server.port}'));
     });
     tearDown(() {
+      if (!requestCompleter.isCompleted) requestCompleter.complete();
       task.cancel();
       server.close();
     });
@@ -254,6 +260,7 @@ void testURLSessionTaskCommon(
 
     test('completed', () async {
       task.resume();
+      requestCompleter.complete();
       while (task.state !=
           NSURLSessionTaskState.NSURLSessionTaskStateCompleted) {
         // Let the event loop run.