Add test for unknown attribute lint

src/test/compile-fail/lint-unknown-attr.rs

+// Copyright 2013 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.
+
+// When denying at the crate level, be sure to not get random warnings from the
+// injected intrinsics by the compiler.
+
+#[deny(attribute_usage)];
+
+#[dance] mod a {} //~ ERROR: unknown attribute
+
+#[dance] fn main() {} //~ ERROR: unknown attribute