switch to xfail for slide count test

extras/test_notebooks.py

@@ -176,12 +176,10 @@ def test_num_slides(file):
 
  notebook = read_notebook(file)
 
- # known issue that these lectures have too many slides
  if file in ["lecture_1.ipynb", "lecture_2.ipynb"]:
- return
- # the various pieces of the lecture can be scaled appropriately
+ pytest.xfail("Known issue that these lectures have too many slides")
  if file == "lecture_6.ipynb":
- return
+ pytest.xfail("The various pieces of the lecture can be scaled appropriately")
 
  columbia = [cell for cell in notebook.cells if "nyu-only" not in get_tags(cell)]
  num_columbia = num_slides(columbia)