Work around dead code warning in test

    warning: struct `Struct` is never constructed
       --> tests/test.rs:338:16
        |
    338 |     pub struct Struct {
        |                ^^^^^^
        |
        = note: `#[warn(dead_code)]` on by default

tests/test.rs

@@ -336,7 +336,7 @@ pub mod issue17 {
     }
 
     pub struct Struct {
-        string: String,
+        pub string: String,
     }
 
     #[async_trait]