Add trailing newlines to files which have no trailing newlines.

src/liballoc/tests/cow_str.rs

@@ -148,4 +148,4 @@ fn check_cow_clone_from() {
     let c2: Cow<str> = Cow::Owned(s);
     c1.clone_from(&c2);
     assert!(c1.into_owned().capacity() >= 25);
-}
+}