Fix test name and simplify helper

src/auth.rs

@@ -87,7 +87,7 @@ mod tests {
     }
 
     #[actix_rt::test]
-    async fn test_login_reports_400_when_user_does_not_exist() {
+    async fn test_login_reports_404_when_user_does_not_exist() {
         let dao = TestUserDao::new();
         dao.create_user("user", "password");