[SPARK-37031][SQL][TESTS][FOLLOWUP] Add a missing test to DescribeNam…

…espaceSuite

### What changes were proposed in this pull request?

This PR proposes to add a missing test on "keeping the legacy output schema" to `DescribeNamespaceSuite`. (#31705 didn't seem to add it).

### Why are the changes needed?

To increase the test coverage.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Added a new test.

Closes #34399 from imback82/SPARK-37031-followup.

Authored-by: Terry Kim <yuminkim@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/core/src/test/scala/org/apache/spark/sql/execution/command/v1/DescribeNamespaceSuite.scala

@@ -19,6 +19,7 @@ package org.apache.spark.sql.execution.command.v1
 
 import org.apache.spark.sql.Row
 import org.apache.spark.sql.execution.command
+import org.apache.spark.sql.internal.SQLConf
 
 /**
  * This base suite contains unified tests for the `DESCRIBE NAMESPACE` command that checks V1
@@ -50,6 +51,23 @@ trait DescribeNamespaceSuiteBase extends command.DescribeNamespaceSuiteBase {
       assert(result(3) === Row("Properties", ""))
     }
   }
+
+  test("Keep the legacy output schema") {
+    Seq(true, false).foreach { keepLegacySchema =>
+      withSQLConf(SQLConf.LEGACY_KEEP_COMMAND_OUTPUT_SCHEMA.key -> keepLegacySchema.toString) {
+        val ns = "db1"
+        withNamespace(ns) {
+          sql(s"CREATE NAMESPACE $ns")
+          val schema = sql(s"DESCRIBE NAMESPACE $ns").schema.fieldNames.toSeq
+          if (keepLegacySchema) {
+            assert(schema === Seq("database_description_item", "database_description_value"))
+          } else {
+            assert(schema === Seq("info_name", "info_value"))
+          }
+        }
+      }
+    }
+  }
 }
 
 /**