doc: add manual test plan for delete backup asynchronously

docs/content/manual/release-specific/v1.8.0/test-delete-backup-asynchronously.md

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+---
+title: Delete Backup Asynchronously
+---
+
+## Related issues
+
+- https://github.com/longhorn/longhorn/issues/8746
+
+## LEP
+
+- https://github.com/longhorn/longhorn/pull/9152
+
+## Test Normal Case (use nfs)
+
+**Given** Longhorn cluster with 3 worker nodes.
+
+**And** Create a volume and attach it.
+
+**And** Write large data to the volume.
+
+**And** Create the first backup from the volume.
+
+**And** Write small data to the volume.
+
+**And** Create a snapshot.
+
+**Then** Delete the first backup and create second backup from the snapshot at the same time.
+
+**Verify** The second backup will be in `Pending` state with message including `waiting for backup to be deleted`.
+
+**Verify** After the first backup is deleted, second backup should be in progress and complete in the end.
+
+
+## Test Backup Delete Error Case (use nfs)
+
+**Given** Longhorn cluster with 3 worker nodes.
+
+**And** Create a volume and attach it.
+
+**And** Write large data to the volume.
+
+**And** Create the first backup from the volume.
+
+**And** Write small data to the volume.
+
+**And** Create a snapshot.
+
+**And** Connect to the backupstore pod and make the backup.cfg file immutable by running the command `$ chattr +i backups/backup_backup-*.cfg`.
+
+**Then** Delete the first backup and create second backup from the snapshot at the same time.
+
+**Verify** The first backup will repeatedly enter the `Deleting` and `Error` states as it attempts to retry the deletion. When in the Error state, an error message related to permissions will be displayed.
+
+**Verify** The second backup will be `InProgress` when the first backup is in `Error` state. The second backup should be complete after awhile.
+
+**Verify** After make the config mutable by running the command `$ chattr -i backups/backup_backup-*.cfg`, after a while, the second backup should be in `Deleting` again and should be deleted successfully.
+
+
+## Test Controller Crashes Case (use nfs)
+
+**Given** Longhorn cluster with 3 worker nodes.
+
+**And** Create a volume and attach it.
+
+**And** Write large data to the volume.
+
+**And** Create the first backup from the volume.
+
+**And** Connect to the backupstore pod and make the backup.cfg file immutable by running the command `$ chattr +i backups/backup_backup-*.cfg`.
+
+**Then** Delete the first backup.
+
+**Verify** The first backup will repeatedly enter the `Deleting` and `Error` states as it attempts to retry the deletion. When in the Error state, an error message related to permissions will be displayed.
+
+**Then** When the backup is in `Deleting` state, delete the longhorn-manager pod which is the owner of the backup. (by checking `Backup.Status.OwnerID`)
+
+**Verify** After the longhorn manager pod is recreated, the backup should turn into `Error` state with message `No deletion in progress record, retry the deletion command`.
+
+**Then** Make the config mutable by running the command `$ chattr -i backups/backup_backup-*.cfg`.
+
+**Verify** The backup should be in `Deleting` again and should be deleted successfully.