[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.
```

Start with acceptance/spanner/backup_test.rb, especially lines 56 and 119, and acceptance/spanner/backup_operations_test.rb at line 44. Re-run the Google Cloud Spanner acceptance tests and inspect how backup restore, deletion, and operation listing are sequenced. Done means the reported backup failures no longer occur reliably in nightly runs.

Escrito por el modelo de indexación a partir del texto del issue.