[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.