Add test for relative path in flats and fix reduced path basename

[Allonck]

• Added test for relative path in get_best_flat helper function
• Added fixes #391

[codecov[bot]]

Codecov Report

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

Project coverage is 79.97%. Comparing base (1ede463) to head (7d55b40).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #392 +/- ## ======================================= Coverage 79.97% 79.97% ======================================= Files 15 15 Lines 2642 2642 ======================================= Hits 2113 2113 Misses 529 529 ```

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