@Override
  public void onSubscribe(Flow.Subscription subscription) {
    // Retrieve all parts
    subscription.request(Long.MAX_VALUE);
  }
Simplifies JDK 11 APIs that don't provide testing utilities to drain a {@link Flow.Publisher} for
easy test-assertions.

<p>Designed to be used solely via {@link FakeSynchronousSubscriber#assertPublishes}.
extensions/data-transfer/portability-data-transfer-apple/src/test/java/org/datatransferproject/datatransfer/apple/http/FakeSynchronousSubscriber.java