[tests] Minor QA improvements

helpers/visceral_make_splits.py

@@ -68,7 +68,7 @@ def save_to_json(
     train_files: list[str],
     test_files: list[str],
     validation_files: list[str],
-    output_file: str,
+    output_file: pathlib.Path,
     organ_1_id: str,
 ):
     data = {
@@ -84,7 +84,7 @@ def save_to_json(
         ],
     }
 
-    with pathlib.Path(output_file).open("w") as json_file:
+    with output_file.open("w") as json_file:
         json.dump(data, json_file, indent=2)