ENH: Add Cuberille tibial + prostate test fixtures

Adds the two ExternalData CID blobs referenced by
InsightSoftwareConsortium/ITK#6302 (Cuberille non-manifold vertex
crash fix; supersedes archived ITKCuberille#80):

- bafkreidqruz73w5mtwqebtmyabuy64eg3fmmc67q6htmxlfgcqu45shwhi
  tibial_cartilage_cropped.nrrd (5622 B)
- bafkreidqilffvaghqnmgaygb3kuri2pmoer35g5dfw524pbuexs2fzhwpu
  prostate_00.nii.gz (3856 B)

Author: hjmjohnson