Ignore new dead_code warnings in test

Since nightly-2024-03-12.

    warning: struct `PackedC` is never constructed
        --> test_suite/tests/test_macros.rs:2237:12
         |
    2237 |     struct PackedC {
         |            ^^^^^^^
         |
         = note: `#[warn(dead_code)]` on by default

    warning: struct `CPacked` is never constructed
        --> test_suite/tests/test_macros.rs:2243:12
         |
    2243 |     struct CPacked {
         |            ^^^^^^^

    warning: struct `CPacked2` is never constructed
        --> test_suite/tests/test_macros.rs:2249:12
         |
    2249 |     struct CPacked2 {
         |            ^^^^^^^^

    warning: struct `Packed2C` is never constructed
        --> test_suite/tests/test_macros.rs:2255:12
         |
    2255 |     struct Packed2C {
         |            ^^^^^^^^

    error: struct `Packed` is never constructed
       --> test_suite/tests/test_gen.rs:733:12
        |
    733 |     struct Packed {
        |            ^^^^^^
        |
    note: the lint level is defined here
       --> test_suite/tests/test_gen.rs:5:9
        |
    5   | #![deny(warnings)]
        |         ^^^^^^^^
        = note: `#[deny(dead_code)]` implied by `#[deny(warnings)]`

    error: struct `Struct` is never constructed
       --> test_suite/tests/test_gen.rs:764:12
        |
    764 |     struct Struct {
        |            ^^^^^^

    error: function `vec_first_element` is never used
       --> test_suite/tests/test_gen.rs:846:4
        |
    846 | fn vec_first_element<T, S>(vec: &[T], serializer: S) -> StdResult<S::Ok, S::Error>
        |    ^^^^^^^^^^^^^^^^^

test_suite/tests/test_macros.rs

@@ -2234,24 +2234,28 @@ fn test_internally_tagged_newtype_variant_containing_unit_struct() {
 fn test_packed_struct_can_derive_serialize() {
     #[derive(Copy, Clone, Serialize)]
     #[repr(packed, C)]
+    #[allow(dead_code)]
     struct PackedC {
         t: f32,
     }
 
     #[derive(Copy, Clone, Serialize)]
     #[repr(C, packed)]
+    #[allow(dead_code)]
     struct CPacked {
         t: f32,
     }
 
     #[derive(Copy, Clone, Serialize)]
     #[repr(C, packed(2))]
+    #[allow(dead_code)]
     struct CPacked2 {
         t: f32,
     }
 
     #[derive(Copy, Clone, Serialize)]
     #[repr(packed(2), C)]
+    #[allow(dead_code)]
     struct Packed2C {
         t: f32,
     }