Ignore dead code lint in tests

New in nightly-2024-03-24 from rust-lang/rust#119552.

    warning: field `0` is never read
      --> tests/test_item.rs:64:37
       |
    64 |                 pub struct S<$life>(pub &$life ());
       |                            -        ^^^^^^^^^^^^^
       |                            |
       |                            field in this struct
    ...
    72 |     m!('a);
       |     ------ in this macro invocation
       |
       = note: `#[warn(dead_code)]` on by default
       = note: this warning originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
    help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
       |
    64 |                 pub struct S<$life>(());
       |                                     ~~

tests/test_item.rs

@@ -61,7 +61,7 @@ mod test_none_delimited_single_lifetime {
     macro_rules! m {
         ($life:lifetime) => {
             paste! {
-                pub struct S<$life>(pub &$life ());
+                pub struct S<$life>(#[allow(dead_code)] pub &$life ());
                 impl<$life> S<$life> {
                     fn f() {}
                 }