text: Title: Tests silently fail if backends do not import cleanly
  Body: Specifically, the line https://github.com/kymatio/kymatio/blob/562e07f55c584522e286b231945f6a63569b490d/kymatio/scattering1d/tests/test_torch_scattering1d.py#L19 (and others like it), have the following consequence: If the `torch_skcuda_backend` module fails to import, the tests succeeds. Now the reason may be the user not having `cupy` installed (which is ok), but it may also be due to a broken backend module. We should catch the latter.