[BUG] address setuptools warnings

[grlee77]

Describe the bug

In a recent CI run there are various warnings related to test folders. For example:

13:14:48 /opt/conda/envs/rapids/conda-bld/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_place/lib/python3.9/site-packages/setuptools/command/build_py.py:153: SetuptoolsDeprecationWarning:     Installing 'cucim.skimage.filters.tests' as data is deprecated, please list it in `packages`.
13:14:48     !!
13:14:48 
13:14:48 
13:14:48     ############################
13:14:48     # Package would be ignored #
13:14:48     ############################
13:14:48     Python recognizes 'cucim.skimage.filters.tests' as an importable package, however it is
13:14:48     included in the distribution as "data".
13:14:48     This behavior is likely to change in future versions of setuptools (and
13:14:48     therefore is considered deprecated).
13:14:48 
13:14:48     Please make sure that 'cucim.skimage.filters.tests' is included as a package by using
13:14:48     setuptools' `packages` configuration field or the proper discovery methods
13:14:48     (for example by using `find_namespace_packages(...)`/`find_namespace:`
13:14:48     instead of `find_packages(...)`/`find:`).
13:14:48 
13:14:48     You can read more about "package discovery" and "data files" on setuptools
13:14:48     documentation page.

~It seems this is related to presence of __init__.py in a subset of the test folders. I think we can just remove these.~

Steps to reproduce

run python setup.py build from the python/cucim folder with a recent release of setuptools