Use pathlib or os.join method and global TEST_PATH variable consistently in test modules

[DaniBodor]

From this comment (see below for examples):

We can either:

Use forward slashes (/) or commas (,) with pathlib functions - both options work. The Path() object will convert forward slashes or commas into the correct kind of slash for the current operating system (nice blog post about it [here](https://medium.com/@ageitgey/python-3-quick-tip-the-easy-way-to-deal-with-file-paths-on-windows-mac-and-linux-11a072b58d5f#:~:text=You%20should%20use%20forward%20slashes,operator%20directly%20in%20your%20code.)).
Use os.path.join() method.

Furthermore: We could adjust the TEST_PATH variable to TESTDATA_PATH and use 1 fewer folder level in the data, as there is no added benefit of having (nearly) identically named folders to the final file.

[github-actions[bot]]

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