fix(backupbackingimage): cleanup backup backing image in the backupstore_cleanup function (backport #1988)

[mergify]

ref: longhorn/longhorn#9041

The issue is caused by the backup backing image left over from the previous test.
We should clean up the backup backing image in the backupstore_cleanup function

---

This is an automatic backport of pull request #1988 done by Mergify.

[mergify]

Cherry-pick of 9a80542 has failed:

On branch mergify/bp/v1.6.x/pr-1988
Your branch is up to date with 'origin/v1.6.x'.

You are currently cherry-picking commit 9a80542c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   manager/integration/tests/backupstore.py
	modified:   manager/integration/tests/common.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   manager/integration/tests/test_system_backup_restore.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[roger-ryao]

The related PR has not been merged to v1.6.x yet: https://github.com/longhorn/longhorn-manager/pull/2824/files. So, this backport needs to wait until it is completed.

[github-actions]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[roger-ryao]

Closing this PR due to longhorn/longhorn-manager#2824 (comment)