fix test bustage due to typo in JS code. This was a known problem that I could have SWORN that shaver checked in a fix for

xpcom/components/nsCategoryManager.js

@@ -245,7 +245,7 @@ CategoryEnumerator.prototype = {
         if (!this.hasMoreElements())
             return null;        // XXX?
         var str = Components.classes["component://netscape/supports-string"]
-            .createInterface(Components.interfaces.nsISupportsString);
+            .createInstance(Components.interfaces.nsISupportsString);
         str.data = this.contents[this.index++];
         return str;
     },