Make headless_defaults example actually headless

Fixes #5260

Author: LegNeato

examples/app/headless_defaults.rs

@@ -1,14 +1,29 @@
 //! An application that runs with default plugins, but without an actual renderer.
 //! This can be very useful for integration tests or CI.
 
-use bevy::{prelude::*, render::settings::WgpuSettings};
+use bevy::{app::AppExit, prelude::*, render::settings::WgpuSettings, window::WindowSettings};
 
 fn main() {
     App::new()
         .insert_resource(WgpuSettings {
             backends: None,
             ..default()
         })
+        .insert_resource(WindowSettings {
+            add_primary_window: false,
+            exit_on_all_closed: false,
+            ..default()
+        })
         .add_plugins(DefaultPlugins)
+        .add_system(do_something)
         .run();
 }
+
+// Normally Bevy exits when all windows are closed.
+//
+// When running in headless mode there are no windows so
+// you must manually send an [`bevy::app::AppExit`] event.
+fn do_something(mut app_exit_events: EventWriter<AppExit>) {
+    info!("Successfully ran! Exiting...");
+    app_exit_events.send(AppExit);
+}