Enrich e2e test to flatten-rbd-pvc command

[satoru-takeuchi]

Related discussion

flatten-rbd-pvc comand only has one e2e test case. It only checks the PVC-PVC clone case. We should check other cases . The additional test cases would be as follows:

• PVC restore: pass without deleting the temporary image corresponding to VolumeSnapshot.
• static PVC: fail
• in-use PVC without allow-in-use option: fail
• in-use PVC with allow-in-use option: pass
• non-cloned/restored PVC: fail
• not-bounded PVC: fail