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

Also doing fmt inplace as requested.

library/std/src/error.rs

@@ -13,6 +13,9 @@
 // coherence challenge (e.g., specialization, neg impls, etc) we can
 // reconsider what crate these items belong in.
 
+#[cfg(test)]
+mod tests;
+
 use core::array;
 use core::convert::Infallible;
 
@@ -738,44 +741,3 @@ impl dyn Error + Send + Sync {
         })
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::Error;
-    use crate::fmt;
-
-    #[derive(Debug, PartialEq)]
-    struct A;
-    #[derive(Debug, PartialEq)]
-    struct B;
-
-    impl fmt::Display for A {
-        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-            write!(f, "A")
-        }
-    }
-    impl fmt::Display for B {
-        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-            write!(f, "B")
-        }
-    }
-
-    impl Error for A {}
-    impl Error for B {}
-
-    #[test]
-    fn downcasting() {
-        let mut a = A;
-        let a = &mut a as &mut (dyn Error + 'static);
-        assert_eq!(a.downcast_ref::<A>(), Some(&A));
-        assert_eq!(a.downcast_ref::<B>(), None);
-        assert_eq!(a.downcast_mut::<A>(), Some(&mut A));
-        assert_eq!(a.downcast_mut::<B>(), None);
-
-        let a: Box<dyn Error> = Box::new(A);
-        match a.downcast::<B>() {
-            Ok(..) => panic!("expected error"),
-            Err(e) => assert_eq!(*e.downcast::<A>().unwrap(), A),
-        }
-    }
-}