code: def load_test_cases():
      cases = []
      for category in ["vegalite", "custom"]:
          category_dir = spec_dir / category
          for file in category_dir.glob("*.vg.json"):
              if file.name not in [
                  # Undefined aggregate column
                  "trellis_area_seattle.vg.json"
              ]:
                  cases.append((category, file.name))
  
      return cases
  
file_path: vegafusion/python/vegafusion/tests/test_pretransform_specs.py
imports: from pathlib import Path
  import json
  import vegafusion
  from vl_convert import vega_to_png
  from io import BytesIO
  import pyarrow
  from skimage.io import imread
  from skimage.metrics import structural_similarity
  import pytest
label_desc: Use image node_id 1 for calling a global function with example usage: load_test_cases() and returns: cases