[Test failures] google-cloud-spanner acceptance tests (flaky)

Failures from the 2022-07-29 nightly run:

```
1) Error:
Spanner Database Backup::spanner#test_0001_creates, get, updates, restore and delete a database backup:
Google::Cloud::FailedPreconditionError: 9:Backup projects/helical-zone-771/instances/google-cloud-ruby-tests/backups/gcruby-220729-4fc0ffcd-crud cannot be deleted at the moment because it is still in use by 1 databases for pending restore activity. Please try deleting the backup once the restore or post-restore optimize operations have completed on these databases. The databases using this backup are displayed in the UI in the Backups table `In use by` column, and returned by the GetBackup API.. debug_error_string:{UNKNOWN:Error received from peer ipv4:74.125.135.95:443 {grpc_message:"Backup projects/helical-zone-771/instances/google-cloud-ruby-tests/backups/gcruby-220729-4fc0ffcd-crud cannot be deleted at the moment because it is still in use by 1 databases for pending restore activity. Please try deleting the backup once the restore or post-restore optimize operations have completed on these databases. The databases using this backup are displayed in the UI in the Backups table `In use by` column, and returned by the GetBackup API.", grpc_status:9, created_time:"2022-07-29T08:43:30.482942649+00:00"}}
/[workspace/google-cloud-spanner-admin-database-v1/lib/google/cloud/spanner/admin/database/v1/database_admin/client.rb:1661](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-spanner-admin-database-v1/lib/google/cloud/spanner/admin/database/v1/database_admin/client.rb?l=1661):in `rescue in delete_backup'
/[workspace/google-cloud-spanner-admin-database-v1/lib/google/cloud/spanner/admin/database/v1/database_admin/client.rb:1623](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-spanner-admin-database-v1/lib/google/cloud/spanner/admin/database/v1/database_admin/client.rb?l=1623):in `delete_backup'
/[workspace/google-cloud-spanner/lib/google/cloud/spanner/service.rb:525](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-spanner/lib/google/cloud/spanner/service.rb?l=525):in `delete_backup'
/[workspace/google-cloud-spanner/lib/google/cloud/spanner/backup.rb:252](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-spanner/lib/google/cloud/spanner/backup.rb?l=252):in `delete'
/[workspace/google-cloud-spanner/acceptance/spanner/backup_test.rb:119](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-spanner/acceptance/spanner/backup_test.rb?l=119):in `block (2 levels) in '

2) Failure:
Spanner Database Backup Operations::spanner#test_0001_list backup operations [/[workspace/google-cloud-spanner/acceptance/spanner/backup_operations_test.rb:44](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-spanner/acceptance/spanner/backup_operations_test.rb?l=44)]:
Expected nil to be a kind of Google::Cloud::Spanner::Backup, not NilClass.
```

from the 2022-07-21 run:

```
1) Failure:
Spanner Database Backup::spanner#test_0001_creates, get, updates, restore and delete a database backup [/[workspace/google-cloud-spanner/acceptance/spanner/backup_test.rb:56](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-spanner/acceptance/spanner/backup_test.rb?l=56)]:
Expected 0 to be > 0.
```

Commencez par acceptance/spanner/backup_test.rb, en particulier les lignes 56 et 119, ainsi que par acceptance/spanner/backup_operations_test.rb à la ligne 44. Relancez les tests d’acceptation Google Cloud Spanner et examinez l’ordre dans lequel sont effectués la restauration des backups, la suppression et le listage des opérations. La tâche est terminée lorsque les échecs de backup signalés ne se produisent plus de manière fiable lors des exécutions nocturnes.

Rédigé par le modèle d'indexation à partir du texte de l'issue.