Use threaded scheduler in tests.

This prevents async I/O timeouts in attach fairings.

Co-authored-by: Jeb Rosen <jeb@jebrosen.com>

core/lib/src/lib.rs

@@ -166,7 +166,9 @@ pub fn custom<T: figment::Provider>(provider: T) -> Rocket {
 #[doc(hidden)]
 pub fn async_test<R>(fut: impl std::future::Future<Output = R> + Send) -> R {
     tokio::runtime::Builder::new()
-        .basic_scheduler()
+        .threaded_scheduler()
+        .thread_name("rocket-test-worker-thread")
+        .core_threads(1)
         .enable_all()
         .build()
         .expect("create tokio runtime")

core/lib/tests/timer-on-attach.rs

@@ -0,0 +1,15 @@
+#[rocket::async_test]
+async fn test_await_timer_inside_attach() {
+
+    async fn do_async_setup() {
+        // By using a timer or I/O resource, we ensure that do_async_setup will
+        // deadlock if no thread is able to tick the time or I/O drivers.
+        rocket::tokio::time::delay_for(std::time::Duration::from_millis(100)).await;
+    }
+
+    rocket::ignite()
+        .attach(rocket::fairing::AdHoc::on_attach("1", |rocket| async {
+            do_async_setup().await;
+            Ok(rocket)
+        }));
+}