Add a test verifying that attempting to manage the same state type twice panics.

core/lib/tests/twice_managed_state.rs

@@ -0,0 +1,7 @@
+struct A;
+
+#[rocket::async_test]
+#[should_panic]
+async fn twice_managed_state() {
+    let _ = rocket::ignite().manage(A).manage(A).inspect().await;
+}