chore: make testing private

Author: noamteyssier

src/error.rs

@@ -294,11 +294,12 @@ where
     }
 }
 
-pub mod testing {
+mod testing {
     #[allow(unused)]
     use super::*;
     use thiserror::Error;
 
+    #[allow(unused)]
     #[derive(Error, Debug)]
     pub enum MyError {
         #[error("Custom error: {0}")]