Add a pretty printer test of impl<T> const Trait

Author: dtolnay

src/test/ui/macros/stringify.rs

@@ -611,6 +611,12 @@ fn test_item() {
         ),
         "pub impl Trait for Struct {}",
     );
+    assert_eq!(
+        stringify_item!(
+            impl<T> const Trait for T {}
+        ),
+        "impl const <T> Trait for T {}", // FIXME
+    );
     assert_eq!(
         stringify_item!(
             impl ~const Struct {}