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

[dongjoon-hyun]

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.

-    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.

[SparkQA]

Test build #62268 has finished for PR 14184 at commit e1fa637.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #62272 has finished for PR 14184 at commit 12e8c94.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

Hi, @liancheng .
Could you review this PR?

It was made by you at 72981bc#diff-e59968489e4f36f43010dd7acd60341dR106 and used in many cases. Recently, SPARK-16459 starts to prevent dropping current database, so we had better update this.

[liancheng]

LGTM, merging to master and branch-2.0. Thanks!

[dongjoon-hyun]

Oh, thank you, @liancheng !