Use explicit buffer length in CopySubBuffer test (#7241)
Use explicit buffer length in CopySubBuffer test (#7241)

IREE_WHOLE_BUFFER isn't supported on CUDA for now. It's
unclear whether we want to support it in the long run.

This also makes the test more targeted and not testing
unrelated features.

iree-org/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,google/iree,google/iree,google/iree,iree-org/iree