improve test output for Categoricals (#18069)

Author: topper-123

doc/source/whatsnew/v0.21.1.txt

@@ -117,7 +117,8 @@ Categorical
 ^^^^^^^^^^^
 
 - Bug in :meth:`DataFrame.astype` where casting to 'category' on an empty ``DataFrame`` causes a segmentation fault (:issue:`18004`)
--
+- Error messages in the testing module have been improved when items have
+  different ``CategoricalDtype`` (:issue:`18069`)
 -
 
 Other

pandas/util/testing.py

@@ -1074,8 +1074,12 @@ def assert_categorical_equal(left, right, check_dtype=True,
 def raise_assert_detail(obj, message, left, right, diff=None):
     if isinstance(left, np.ndarray):
         left = pprint_thing(left)
+    elif is_categorical_dtype(left):
+        left = repr(left)
     if isinstance(right, np.ndarray):
         right = pprint_thing(right)
+    elif is_categorical_dtype(right):
+        right = repr(right)
 
     msg = """{obj} are different