Exclude `n_slices.txt` from comparison

Because `$tests_old_dir/Extraction_eddies_region_5/SHPC` is opened
with `SHPC_class` in test `Plot_eddy_contours_window`, which will
create `n_slices.txt`, so there will always be this file only in
`$tests_old_dir/Extraction_eddies_region_5/SHPC`.