Allow the staged_api

src/libtest/lib.rs

@@ -12,6 +12,7 @@
 #![unstable(feature = "test", issue = "27812")]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/",
        test(attr(deny(warnings))))]
+#![feature(staged_api)]
 #![feature(test)]
 
 extern crate libtest;