How to load assets in tests that while using app.update ?

[Maykeye]

#[test]ing. I'm trying to test things with I/O and can't get it work without relying on app.run():

Here's dirty PoC.

#[test]
fn test_loading_image() {
    fn headless() -> PluginGroupBuilder {
        let mut winit = WinitPlugin::<WakeUp>::default();
        winit.run_on_any_thread = true;
        DefaultPlugins
            .set(RenderPlugin {
                render_creation: WgpuSettings {
                    backends: None,
                    ..default()
                }
                .into(),
                ..default()
            })
            .set(winit)
            .disable::<AudioPlugin>()
    }

    let mut app = App::new();
    app.add_plugins(headless());

    #[derive(Resource)]
    struct Timeout(Timer);
    #[derive(Resource)]
    struct Sample(Handle<Image>);
    app.insert_resource(Timeout(Timer::from_seconds(1.0, TimerMode::Once)));

    app.add_systems(Startup, |mut cmd: Commands, ass: Res<AssetServer>| {
        cmd.insert_resource(Sample(ass.load("sample.png")));
    });

    fn tick_timeout(
        mut t: ResMut<Timeout>,
        time: Res<Time<Real>>,
        sample: Res<Sample>,
        asset_serer: Res<AssetServer>,
        images: Res<Assets<Image>>,
    ) {
        let load_state = asset_serer.load_state(&sample.0);
        let from_load_state = matches!(load_state, bevy::asset::LoadState::Loaded);
        let from_assets = images.get(&sample.0).is_some();
        t.0.tick(time.delta());
        if t.0.just_finished() {
            println!(">>>>");
            println!(">>>> finished timer with {from_load_state} {from_assets}");
            println!(">>>>");
            exit(1);
        }
    }
    app.add_systems(Update, tick_timeout);

    //app.run();
    /*
     * WORKS:
     * >>>> finished timer with true true
     */

    /* DOES NOT WORK
     * >>>> finished timer with false false
     */
    for _ in 0..1000000 {
        app.update();
        std::thread::sleep(Duration::from_secs_f32(0.01));
    }
    assert_eq!(1,2);
}

I expect test to fail(duh) with report that image is loaded.
If I do it with app.run(), it does -- both assets and asset_server report it's true that image was loaded.

However just using app.update() doesn't seems to load anything. What to do?