new_contents: import cv2
  import numpy as np
  from plantcv.plantcv import invert
  
  
  def test_invert(test_data):
      # Read in test data
      img = cv2.imread(test_data.small_bin_img, -1)
      inverted_img = invert(gray_img=img)
      total_px = img.shape[0] * img.shape[1]
      expected = total_px - np.count_nonzero(img)
      assert np.count_nonzero(inverted_img) == expected
  
subject: Add invert tests module