RxJava - 测试subscribeOn（）

Question

我想在我的项目中使用RxJava并编写简单的异步方法，现在我尝试测试它。我无法使用Executor对其进行测试，因为我得到了结果：java.lang.AssertionError: Not completed! (0 completions)

服务方式：

    @Override
    public Observable<User> addOrderAsynchronously(String username, Order order) {
        if (username != null && order != null) {
            return Observable
                    .fromCallable(() -> userRepository.findByUsername(username))
                    .filter(Objects::nonNull)
                    .map(user -> {
                        synchronized (this) {
                             user.getOrders().add(order);
                             order.setUser(user);
                        }
                         return user;
                    })
                    .map(userRepository::save);
    }
    return Observable.empty();
}

没有执行者的工作测试：

@Test
public void shouldSaveUserSynchronouslyTest() throws Exception {
    // given
    TestSubscriber<User> subscriber = new TestSubscriber<>();
    final Order order = new Order();
    final User user = new User();

    when(userRepository.findByUsername(user.getUsername()))
            .thenReturn(user);

    when(userRepository.save(any(User.class)))
            .thenReturn(user);

    // when
    userService.addOrderAsynchronously(user.getUsername(), order)
            .subscribe(subscriber);

    // then
    subscriber.assertCompleted();
    subscriber.assertNoErrors();
    assertThat(subscriber.getOnNextEvents()).isEqualTo(Collections.singletonList(user));
}

现在我想用执行者测试：

@Test
    public void shouldSaveUserSynchronouslyTest() throws Exception {
        // given
        TestSubscriber<User> subscriber = new TestSubscriber<>();
        final Order order = new Order();
        final User user = new User();

        when(userRepository.findByUsername(user.getUsername()))
                .thenReturn(user);

        when(userRepository.save(any(User.class)))
                .thenReturn(user);

        // when
        userService.addOrderAsynchronously(user.getUsername(), order)
                .subscribeOn(Schedulers.from(executor))
                .subscribe(subscriber);

        // then
        subscriber.assertCompleted();
        subscriber.assertNoErrors();
        assertThat(subscriber.getOnNextEvents()).isEqualTo(Collections.singletonList(user));
    }

此测试无效，因为我得到结果java.lang.AssertionError: Not completed! (0 completions)。

执行者来自@Bean方法：

@Bean
public Executor executor() {
    final ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
    executor.setCorePoolSize(8);
    executor.setMaxPoolSize(8);
    executor.setQueueCapacity(500);
    executor.initialize();
    return executor;
}

如何用执行程序编写单元测试？