fix test case because if a plan containes the exact same instance of Exchange multiple times then it makes no sense to replace one of the instances to a ReusedExchange

Author: peter-toth

sql/core/src/test/scala/org/apache/spark/sql/execution/PlannerSuite.scala

@@ -470,7 +470,7 @@ class PlannerSuite extends SharedSparkSession {
       Inner,
       None,
       shuffle,
-      shuffle)
+      shuffle.copy())
 
     val outputPlan = ReuseExchange(spark.sessionState.conf).apply(inputPlan)
     if (outputPlan.collect { case e: ReusedExchangeExec => true }.size != 1) {