Work around dead_code warning in tests

    warning: field `0` is never read
     --> tests/helper/lib.rs:5:19
      |
    5 | pub struct Struct(str);
      |            ------ ^^^
      |            |
      |            field in this struct
      |
      = note: `#[warn(dead_code)]` on by default
    help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
      |
    5 | pub struct Struct(());
      |                   ~~

    warning: field `0` is never read
     --> tests/test_custom.rs:7:23
      |
    7 |     pub struct Custom(str);
      |                ------ ^^^
      |                |
      |                field in this struct
      |
      = note: `#[warn(dead_code)]` on by default
    help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
      |
    7 |     pub struct Custom(());
      |                       ~~

tests/helper/lib.rs

@@ -2,4 +2,4 @@ use ref_cast::RefCastCustom;
 
 #[derive(RefCastCustom)]
 #[repr(transparent)]
-pub struct Struct(str);
+pub struct Struct(#[allow(dead_code)] str);

tests/test_custom.rs

@@ -4,7 +4,7 @@ mod forbid_unsafe {
 
     #[derive(RefCastCustom)]
     #[repr(transparent)]
-    pub struct Custom(str);
+    pub struct Custom(#[allow(dead_code)] str);
 
     impl Custom {
         #[ref_cast_custom]