Fix tidy by adding new feature gate test

Author: Esteban Küber

src/test/ui/feature-gate-format_args_nl.rs

@@ -0,0 +1,13 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    format_args_nl!(""); //~ ERROR `format_args_nl` is only for internal language use
+}

src/test/ui/feature-gate-format_args_nl.stderr

@@ -0,0 +1,11 @@
+error[E0658]: `format_args_nl` is only for internal language use and is subject to change
+  --> $DIR/feature-gate-format_args_nl.rs:12:5
+   |
+LL |     format_args_nl!(""); //~ ERROR `format_args_nl` is only for internal language use
+   |     ^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(format_args_nl)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.