enable a test that was accidentally left disabled

Author: RalfJung

tests/run-pass/drop_empty_slice.rs

@@ -1,9 +1,7 @@
 #![feature(box_syntax)]
-// This disables the test completely:
-// ignore-stage1
 
 fn main() {
-    // With the nested Vec, this is calling Offset(Unique::empty(), 0).
+    // With the nested Vec, this is calling Offset(Unique::empty(), 0) on drop.
     let args : Vec<Vec<i32>> = Vec::new();
-    let local = box args;
+    let _ = box args;
 }