Cython tests need build instructions

[vzhurba01]

Currently the Cython tests are segregated to test_cython and there are no instructions on how to build them.
Should we have a setup.py? Can we avoid it with a simply cythonize + command approach?

Need updates to README for how to build them and use them.
Also, would it be better to have a folder layout such that we have:

• cuda_bindings/tests/python/* for Python tests
• cuda_bindings/tests/cython/* for Cython tests

[leofang]

Can we avoid it with a simply cythonize + command approach?

Yes I think so. I like doing things in a simple & easy-to-maintain way even if it might seem "silly", and this is one such case. We can just make Cython a test requirement (gcc/msvc are needed too, but we can't list them in requirements.txt or pyproject.toml...) and we can call cythonize() in a subprocess, similar to what we do in conda-forge.
https://github.com/conda-forge/cuda-python-feedstock/blob/4a12ae297e8ebf4784269fb8daa89a4d0a0a417f/recipe/meta.yaml#L95-L98

[leofang]

We also need the CUDA headers of the same major.minor as test dependencies (xref: #245), ex:
https://github.com/conda-forge/cuda-python-feedstock/blob/4a12ae297e8ebf4784269fb8daa89a4d0a0a417f/recipe/meta.yaml#L86-L89