std: move "mod tests/benches" to separate files

Also doing fmt inplace as requested.

library/std/src/io/error.rs

@@ -1,3 +1,6 @@
+#[cfg(test)]
+mod tests;
+
 use crate::convert::From;
 use crate::error;
 use crate::fmt;
@@ -574,60 +577,3 @@ fn _assert_error_is_sync_send() {
     fn _is_sync_send<T: Sync + Send>() {}
     _is_sync_send::<Error>();
 }
-
-#[cfg(test)]
-mod test {
-    use super::{Custom, Error, ErrorKind, Repr};
-    use crate::error;
-    use crate::fmt;
-    use crate::sys::decode_error_kind;
-    use crate::sys::os::error_string;
-
-    #[test]
-    fn test_debug_error() {
-        let code = 6;
-        let msg = error_string(code);
-        let kind = decode_error_kind(code);
-        let err = Error {
-            repr: Repr::Custom(box Custom {
-                kind: ErrorKind::InvalidInput,
-                error: box Error { repr: super::Repr::Os(code) },
-            }),
-        };
-        let expected = format!(
-            "Custom {{ \
-             kind: InvalidInput, \
-             error: Os {{ \
-             code: {:?}, \
-             kind: {:?}, \
-             message: {:?} \
-             }} \
-             }}",
-            code, kind, msg
-        );
-        assert_eq!(format!("{:?}", err), expected);
-    }
-
-    #[test]
-    fn test_downcasting() {
-        #[derive(Debug)]
-        struct TestError;
-
-        impl fmt::Display for TestError {
-            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                f.write_str("asdf")
-            }
-        }
-
-        impl error::Error for TestError {}
-
-        // we have to call all of these UFCS style right now since method
-        // resolution won't implicitly drop the Send+Sync bounds
-        let mut err = Error::new(ErrorKind::Other, TestError);
-        assert!(err.get_ref().unwrap().is::<TestError>());
-        assert_eq!("asdf", err.get_ref().unwrap().to_string());
-        assert!(err.get_mut().unwrap().is::<TestError>());
-        let extracted = err.into_inner().unwrap();
-        extracted.downcast::<TestError>().unwrap();
-    }
-}