Replace (TF)CommonTestCases for modeling with a mixin.

I suspect the wrapper classes were created in order to prevent the
abstract base class (TF)CommonModelTester from being included in test
discovery and running, because that would fail.

I solved this by replacing the abstract base class with a mixin.

Code changes are just de-indenting and automatic reformattings
performed by black to use the extra line space.

tests/test_modeling_tf_xlnet.py

@@ -15,11 +15,12 @@
 from __future__ import absolute_import, division, print_function
 
 import random
+import unittest
 
 from transformers import XLNetConfig, is_tf_available
 
 from .test_configuration_common import ConfigTester
-from .test_modeling_tf_common import TFCommonTestCases, ids_tensor
+from .test_modeling_tf_common import TFModelTesterMixin, ids_tensor
 from .utils import CACHE_DIR, require_tf, slow
 
 
@@ -37,7 +38,7 @@ if is_tf_available():
 
 
 @require_tf
-class TFXLNetModelTest(TFCommonTestCases.TFCommonModelTester):
+class TFXLNetModelTest(TFModelTesterMixin, unittest.TestCase):
 
     all_model_classes = (
         (