[SPARK-21913][SQL][TEST] withDatabase` should drop database with CASCADE

## What changes were proposed in this pull request?

Currently, `withDatabase` fails if the database is not empty. It would be great if we drop cleanly with CASCADE.

## How was this patch tested?

This is a change on test util. Pass the existing Jenkins.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #19125 from dongjoon-hyun/SPARK-21913.

sql/core/src/test/scala/org/apache/spark/sql/test/SQLTestUtils.scala

@@ -247,7 +247,7 @@ private[sql] trait SQLTestUtils
   protected def withDatabase(dbNames: String*)(f: => Unit): Unit = {
     try f finally {
       dbNames.foreach { name =>
-        spark.sql(s"DROP DATABASE IF EXISTS $name")
+        spark.sql(s"DROP DATABASE IF EXISTS $name CASCADE")
       }
       spark.sql(s"USE $DEFAULT_DATABASE")
     }