Add compile-fail test for the new feature gate

src/test/compile-fail/feature-gate-allow_fail.rs

@@ -0,0 +1,17 @@
+// Copyright 2015 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.
+
+// check that #[allow_fail] is feature-gated
+
+#[allow_fail] //~ ERROR allow_fail attribute is currently unstable
+fn ok_to_fail() {
+    assert!(false);
+}
+

src/test/run-pass/test-allow-fail-attr.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 // compile-flags: --test
+#![feature(allow_fail)]
 
 #[test]
 #[allow_fail]