Allow interior_mutable_const_item_mutations in tests

Author: Urgau

tests/ui/consts/issue-17718.rs

@@ -1,7 +1,9 @@
 //@ run-pass
-#![allow(dead_code)]
 //@ aux-build:issue-17718-aux.rs
 
+#![allow(dead_code)]
+#![allow(interior_mutable_const_item_mutations)]
+
 extern crate issue_17718_aux as other;
 
 use std::sync::atomic::{AtomicUsize, Ordering};