Enable iterator-helpers test262 feature (#546)

Enable the feature to give us a burndown list of things to implement.

Author: bnoordhuis

test262.conf

@@ -140,7 +140,7 @@ Intl.NumberFormat-v3=skip
 Intl.RelativeTimeFormat=skip
 Intl.Segmenter=skip
 IsHTMLDDA
-iterator-helpers=skip
+iterator-helpers
 json-modules=skip
 json-parse-with-source=skip
 json-superset