Fix SQLListenerMemoryLeakSuite test error

Author: carsonwang

sql/core/src/test/scala/org/apache/spark/sql/execution/ui/SQLListenerSuite.scala

@@ -343,6 +343,8 @@ class SQLListenerMemoryLeakSuite extends SparkFunSuite {
       .set("spark.sql.ui.retainedExecutions", "50") // Set it to 50 to run this test quickly
     val sc = new SparkContext(conf)
     try {
+      // Clear the sql listener created by a previous test suite.
+      SQLContext.clearSqlListener()
       val sqlContext = new SQLContext(sc)
       import sqlContext.implicits._
       // Run 100 successful executions and 100 failed executions.