update testdata to use a single model source

[k-dominik]

Currently we have separate folders for the pytorch, onnx and torchscript flavor for tests in tests/data. This is a bit unfortunate - on spec changes, each of them have to be altered separately and the hierarchy/history is not clear. There is a joint model here: https://github.com/bioimage-io/pytorch-bioimage-io/blob/master/specs/models/unet2d_nuclei_broad/UNet2DNucleiBroad.model.yaml

see @constantinpape's comment here: #164 (comment)

[constantinpape]

Might be relevant here: @FynnBe and me have discussed restructuring the whole bioimage.io code a bit and also move parts of tiktorch in the process, see bioimage-io/core-bioimage-io-python#72.