Add new test to check invert method

tests/utils/augmentor_utils_test.py

+import cv2
+import numpy as np
+
+from unagi.utils.augmentor_utils import InvertPartAugmentor
+
+
+def test_invert(bin_img_path):
+    """Test the __invert__ method of InvertPartAugmentor"""
+    probability = 0.5  # probability of operation being performed
+    augmentor = InvertPartAugmentor(probability)
+
+    img = img = cv2.imread(bin_img_path, cv2.IMREAD_GRAYSCALE)
+    inverted_img = augmentor.__invert__(img)
+    inverted_img_np = np.array(inverted_img)
+
+    assert inverted_img_np is not None
+    assert inverted_img_np.shape == img.shape
+    assert inverted_img_np.dtype == img.dtype