Work around new dead code warning in test

warning: struct `StructTransparentGeneric` is never constructed
       --> tests/test_generics.rs:161:12
        |
    161 | pub struct StructTransparentGeneric<E>(E);
        |            ^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: `#[warn(dead_code)]` on by default

tests/test_generics.rs

@@ -158,4 +158,4 @@ pub struct StructFromGeneric<E> {
 //
 #[derive(Error, Debug)]
 #[error(transparent)]
-pub struct StructTransparentGeneric<E>(E);
+pub struct StructTransparentGeneric<E>(pub E);