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 49.86%. Comparing base (85b3f55) to head (b57995e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4567   +/-   ##
=======================================
  Coverage   49.86%   49.86%           
=======================================
  Files         151      151           
  Lines       29867    29867           
=======================================
  Hits        14892    14892           
  Misses      14975    14975           

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