fix some cfail tests

Author: Jorge Aparicio

src/test/compile-fail/issue-17999.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![deny(unused_variables)]
+#![feature(core)]
 
 fn main() {
     for _ in 1is..101 {

src/test/compile-fail/liveness-unused.rs

@@ -11,6 +11,7 @@
 #![deny(unused_variables)]
 #![deny(unused_assignments)]
 #![allow(dead_code, non_camel_case_types)]
+#![feature(core)]
 #![feature(os)]
 
 fn f1(x: isize) {