[SPARK-16529][SQL][TEST] `withTempDatabase` should set `default` database before dropping

## What changes were proposed in this pull request?

`SQLTestUtils.withTempDatabase` is a frequently used test harness to setup a temporary table and clean up finally. This issue improves like the following for usability.

```scala
-    try f(dbName) finally spark.sql(s"DROP DATABASE $dbName CASCADE")
+    try f(dbName) finally {
+      if (spark.catalog.currentDatabase == dbName) {
+        spark.sql(s"USE ${DEFAULT_DATABASE}")
+      }
+      spark.sql(s"DROP DATABASE $dbName CASCADE")
+    }
```

In case of forgetting to reset the databaes, `withTempDatabase` will not raise Exception.

## How was this patch tested?

This improves test harness.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14184 from dongjoon-hyun/SPARK-16529.

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

@@ -29,6 +29,7 @@ import org.scalatest.BeforeAndAfterAll
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.sql._
 import org.apache.spark.sql.catalyst.analysis.NoSuchTableException
+import org.apache.spark.sql.catalyst.catalog.SessionCatalog.DEFAULT_DATABASE
 import org.apache.spark.sql.catalyst.FunctionIdentifier
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.util._
@@ -196,7 +197,12 @@ private[sql] trait SQLTestUtils
       fail("Failed to create temporary database", cause)
     }
 
-    try f(dbName) finally spark.sql(s"DROP DATABASE $dbName CASCADE")
+    try f(dbName) finally {
+      if (spark.catalog.currentDatabase == dbName) {
+        spark.sql(s"USE ${DEFAULT_DATABASE}")
+      }
+      spark.sql(s"DROP DATABASE $dbName CASCADE")
+    }
   }
 
   /**