[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 <top (required)>'

  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.

Bắt đầu với acceptance/spanner/backup_test.rb, đặc biệt là các dòng 56 và 119, cùng với acceptance/spanner/backup_operations_test.rb ở dòng 44. Chạy lại các bài kiểm thử chấp nhận của Google Cloud Spanner và kiểm tra thứ tự thực hiện của việc khôi phục backup, xóa và liệt kê operation. Hoàn tất khi các lỗi backup đã báo cáo không còn xảy ra một cách ổn định trong các lần chạy hằng đêm.