[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.
```

acceptance/spanner/backup_test.rb、特に 56 行目と 119 行目、および acceptance/spanner/backup_operations_test.rb の 44 行目から始めてください。Google Cloud Spanner の受け入れテストを再実行し、バックアップの復元、削除、オペレーションの一覧表示がどのような順序で行われるかを調べてください。報告されたバックアップの失敗が夜間実行で確実には発生しなくなれば完了です。

索引モデルが issue の本文から書いたものです。