Fix GroupWindowStringExpressionTest testcase bug

 // String / Java API
    val resJava = t
      .window(JSlide.over("4.hours").every("2.hours").on("rowtime") as 'w)
      .groupBy("w, string")
      .select(
        "string, " +
        "myCountFun(string), " +
        "int.sum, " +
        "weightAvgFun(long, int), " +
        "weightAvgFun(int, int) * 2, " +
        "start(w)," +
        "end(w)")

org.apache.flink.table.api.ValidationException: Cannot resolve [w] given input [int, long, string, rowtime].

	at org.apache.flink.table.plan.logical.LogicalNode.failValidation(LogicalNode.scala:143)
	at org.apache.flink.table.plan.logical.LogicalNode$$anonfun$validate$1.applyOrElse(LogicalNode.scala:86)
	at org.apache.flink.table.plan.logical.LogicalNode$$anonfun$validate$1.applyOrElse(LogicalNode.scala:83)
	at org.apache.flink.table.plan.TreeNode.postOrderTransform(TreeNode.scala:72)
	at org.apache.flink.table.plan.logical.LogicalNode.org$apache$flink$table$plan$logical$LogicalNode$$expressionPostOrderTransform$1(LogicalNode.scala:119)
	at org.apache.flink.table.plan.logical.LogicalNode$$anonfun$7$$anonfun$apply$1.apply(LogicalNode.scala:132)
	at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
	at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
	at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)