Ignore new dead_code warnings in test

Since nightly-2024-03-12.

    warning: struct `S` is never constructed
       --> tests/test.rs:366:12
        |
    366 |     struct S {}
        |            ^
        |
        = note: `#[warn(dead_code)]` on by default

    warning: struct `Struct` is never constructed
        --> tests/test.rs:1492:12
         |
    1492 |     struct Struct;
         |            ^^^^^^

tests/test.rs

@@ -363,6 +363,7 @@ pub mod issue23 {
         }
     }
 
+    #[allow(dead_code)]
     struct S {}
 
     #[async_trait]
@@ -1489,6 +1490,7 @@ pub mod issue226 {
         async fn cfg_param_tuple(&self, (left, right): (u8, u8));
     }
 
+    #[allow(dead_code)]
     struct Struct;
 
     #[async_trait]