os.path and pathlib

There is a mix of os.path.join() calls and simple string concatenation throughout the code. Examples: getTests() versus make_datadir_subdirectory() in TorNet.py (also, just noticed the mixed camelCase and snake_case which might deserve its own style ticket).

I think we want to be consistent with path handling, but has pathlib been taken into consideration to make it more self-documenting where Chutney needs to deal with paths?