Fix test issue on s390x

Anytime the TIFF RGBA interface is involved, the endianness must be
taken into account.

Closes: #654

tests/test_libtiff.py

@@ -1,6 +1,7 @@
 # standard library imports
 import importlib.resources as ir
 import platform
+import sys
 import unittest
 from unittest.mock import patch
 import warnings
@@ -119,6 +120,9 @@ def test_bigtiff_ycbcr_2x2_tiled(self):
         actual = libtiff.readRGBAImageOriented(fp)
         libtiff.close(fp)
 
+        # Adjust for big-endian if necessary
+        actual = np.flip(actual, 2) if sys.byteorder == 'big' else actual
+
         error = fixtures.skimage.metrics.mean_squared_error(
             actual[:, :, :3], expected
         )