core: set pv name correctly on subvolume test

[satoru-takeuchi]

Description of your changes:

PVC.spec.volumeName is empty if it's not bound to any PV. This problem can be avoided by getting the name after verifying PVC to be bound.

ref. https://github.com/rook/kubectl-rook-ceph/actions/runs/8908445423/job/24464082185?pr=229#step:15:128

+ kubectl get pvc cephfs-pvc-retain
NAME                STATUS    VOLUME   CAPACITY   ACCESS MODES   STORAGECLASS         AGE
cephfs-pvc-retain   Pending                                      rook-cephfs-retain   0s
++ kubectl get pvc cephfs-pvc-retain '-o=jsonpath={.spec.volumeName}'
+ : ''
+ wait_for_pvc_to_be_bound_state_default

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• [x] Documentation has been updated, if necessary.
• [x] Unit tests have been added, if necessary.
• [x] Integration tests have been added, if necessary.