Fix volume backup test

This patch is to remove redundant waiters which being used twice in
VolumesBackupsAdminTest.test_volume_backup_export_import and add status
check for backup after being restored.

Change-Id: Idf26178325325d4566c2f718b638770ce090edb4
diff --git a/tempest/api/volume/admin/test_volumes_backup.py b/tempest/api/volume/admin/test_volumes_backup.py
index 35eea11..c179c35 100644
--- a/tempest/api/volume/admin/test_volumes_backup.py
+++ b/tempest/api/volume/admin/test_volumes_backup.py
@@ -105,16 +105,18 @@
         self.addCleanup(self.volumes_client.delete_volume,
                         restore['volume_id'])
         self.assertEqual(backup['id'], restore['backup_id'])
-        waiters.wait_for_volume_resource_status(self.volumes_client,
-                                                restore['volume_id'],
-                                                'available')
+
+        # When restore operation is performed then, backup['id']
+        # goes to 'restoring' state so we need to wait for
+        # backup['id'] to become 'available'.
+        waiters.wait_for_volume_resource_status(
+            self.backups_client, backup['id'], 'available')
+        waiters.wait_for_volume_resource_status(
+            self.volumes_client, restore['volume_id'], 'available')
 
         # Verify if restored volume is there in volume list
         volumes = self.volumes_client.list_volumes()['volumes']
         self.assertIn(restore['volume_id'], [v['id'] for v in volumes])
-        waiters.wait_for_volume_resource_status(self.admin_backups_client,
-                                                import_backup['id'],
-                                                'available')
 
     @decorators.idempotent_id('47a35425-a891-4e13-961c-c45deea21e94')
     def test_volume_backup_reset_status(self):