Test list backup when cluster has node cordoned before Longhorn installation

https://github.com/longhorn/longhorn/issues/7619

Test step

Given a cluster has 3 worker nodes.
And 2 worker nodes are cordoned.
And Longhorn is installed.

When Setting up a backup target.

Then no error is observed on the UI Backup page.
And Backup custom resources are created if the backup target has existing backups.

[Edit]