Change set_filenames in single_run fixture to use file name instead of folder path

[clmould]

Description

Previously this test was using the tmp_path in set_filenames, leading to files being created with the name of the test inside a folder of the same name:

This change means the files created during a run of pytest now have the name of the file preserved:

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.85%. Comparing base (85b3f55) to head (53ce3a0).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4567      +/-   ##
==========================================
- Coverage   49.86%   49.85%   -0.01%     
==========================================
  Files         151      151              
  Lines       29867    29875       +8     
==========================================
+ Hits        14892    14894       +2     
- Misses      14975    14981       +6     

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