message: [tf.data service] Fix flaky GcUnusedJob test.
  The dataset needs more than 10 elements because by default we prefetch 16, creating a race condition where the job may finish before being garbage collected, resulting in self.assertEqual(worker._num_tasks(), 1) failing because there are 0 tasks.
diff: @@ -547,12 +547,12 @@ class DataServiceOpsTest(test_base.DatasetTestBase, parameterized.TestCase):
  dispatcher = self.start_dispatch_server(
  job_gc_check_interval_ms=50, job_gc_timeout_ms=20)
  worker = self.start_worker_server(dispatcher) # pylint: disable=unused-variable
  - num_elements = 10
  + num_elements = 100
  ds = _make_distributed_range_dataset(
  num_elements, dispatcher, job_name=job_name)
  it = iter(ds)
  - self.assertEqual(0, next(it).numpy())
  - self.assertEqual(1, worker._num_tasks())
  + self.assertEqual(next(it).numpy(), 0)
  + self.assertEqual(worker._num_tasks(), 1)
  del it
  while worker._num_tasks() > 0:
  time.sleep(0.1)
  