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

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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[hombit]

Great!