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

[dazuma]

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.

[bajajneha27]

Another failure on 2022-01-12.