Add tests for Readers which run via pytests

[pgleeson]

Started at https://github.com/openworm/ConnectomeToolbox/blob/feature/pytest/cect/tests/test_varshney.py in feature/pytest branch.

[nanoman657]

I have made some good progress on debugging why test_varshney.py doesn't run on MacOS. I hope to have a PR to fix the bug in about a week.

[pgleeson]

@nanoman657 As mentioned in the PR, do try out the latest main locally, as that should pass fine on MacOS.

[nanoman657]

@pgleeson Sounds good. I anticipate that it will pass. If this is a good issue to make PRs towards, I'll add some more PRs to test the readers.