fix: Flaky 'allTasks' integration test due to race condition

The integration test 'allTasks and allTaskIds with allGroups set to true'
in `general_test.dart` was flaky because the default download task
(using `https://google.com`) completed too quickly, often finishing
before the `allTasks()` assertion could execute. This resulted in an
empty list of tasks and a test failure.

This change updates the test to use `urlWithContentLength` (a ~5MB file)
to ensure the download takes sufficient time, keeping the tasks in an
active state (enqueued/running) during the assertions.

Addresses the race condition where `allTasks` returned 0 active tasks
instead of the expected 2.

Author: google-labs-jules[bot]

example/integration_test/general_test.dart

@@ -549,6 +549,7 @@ void main() {
         (widgetTester) async {
       print('Starting allTasks with allGroups set to true');
       FileDownloader().registerCallbacks(taskStatusCallback: statusCallback);
+      task = DownloadTask(url: urlWithContentLength, filename: defaultFilename);
       final task2 = task.copyWith(group: 'group2');
       expect(await FileDownloader().enqueue(task), isTrue);
       expect(await FileDownloader().enqueue(task2), isTrue);