Use pathlib for pytest fixtures.

[delucchi-cmu]

Change Description

This PR touches only files under tests/, and changes code to use pathlib.Path for constructing test data paths, instead of os.path.join.

This shouldn't change any functionality, but is just cleaner.

Code Quality

• [x] I have read the Contribution Guide and LINCC Frameworks Code of Conduct
• [x] My code follows the code style of this project
• [x] My code builds (or compiles) cleanly without any errors or warnings
• [x] My code contains relevant comments and necessary documentation

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.78%. Comparing base (3bcec06) to head (4439aca). Report is 41 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #333 +/- ## ======================================= Coverage 99.78% 99.78% ======================================= Files 26 26 Lines 1389 1389 ======================================= Hits 1386 1386 Misses 3 3 ```

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