Eliminate unused variable warning

Author: fintelia

src/libstd/collections/hash/map.rs

@@ -3532,7 +3532,7 @@ mod test_map {
             m.insert(x, ());
         }
 
-        for i in 0..1000 {
+        for _ in 0..1000 {
             let x = rng.gen_range(-10, 10);
             match m.entry(x) {
                 Vacant(_) => {}