Fix failing test

Author: Stjepan Glavina

src/libcore/tests/iter.rs

@@ -1,3 +1,4 @@
+use core::cell::Cell;
 use core::iter::*;
 use core::{i8, i16, isize};
 use core::usize;
@@ -1908,19 +1909,19 @@ fn test_once() {
 
 #[test]
 fn test_once_with() {
-    let mut count = 0;
+    let mut count = Cell::new(0);
     let mut it = once_with(|| {
-        count += 1;
+        count.set(count.get() + 1);
         42
     });
 
-    assert_eq!(count, 0);
+    assert_eq!(count.get(), 0);
     assert_eq!(it.next(), Some(42));
-    assert_eq!(count, 1);
+    assert_eq!(count.get(), 1);
     assert_eq!(it.next(), None);
-    assert_eq!(count, 1);
+    assert_eq!(count.get(), 1);
     assert_eq!(it.next(), None);
-    assert_eq!(count, 1);
+    assert_eq!(count.get(), 1);
 }
 
 #[test]