Add missing test for groupBy

Author: Andrew Or

core/src/test/scala/org/apache/spark/util/ClosureCleanerSuite.scala

@@ -88,6 +88,7 @@ class ClosureCleanerSuite extends FunSuite {
       expectCorrectException { TestUserClosuresActuallyCleaned.testFlatMap(rdd) }
       expectCorrectException { TestUserClosuresActuallyCleaned.testFilter(rdd) }
       expectCorrectException { TestUserClosuresActuallyCleaned.testSortBy(rdd) }
+      expectCorrectException { TestUserClosuresActuallyCleaned.testGroupBy(rdd) }
       expectCorrectException { TestUserClosuresActuallyCleaned.testKeyBy(rdd) }
       expectCorrectException { TestUserClosuresActuallyCleaned.testMapPartitions(rdd) }
       expectCorrectException { TestUserClosuresActuallyCleaned.testMapPartitionsWithIndex(rdd) }