core tests: Add use vec; directive inside core::hashmap tests

This is probably to work around a bug.

src/libcore/hashmap.rs

@@ -668,6 +668,7 @@ pub fn test_expand() {
 #[test]
 mod test_set {
     use super::*;
+    use vec;
 
     #[test]
     fn test_disjoint() {