Remove unittest.main() in test modules.

This construct isn't used anymore these days.

Running python tests/test_foo.py puts the tests/ directory on
PYTHONPATH, which isn't representative of how we run tests.

Use python -m unittest tests/test_foo.py instead.

tests/test_modeling_distilbert.py

@@ -14,8 +14,6 @@
 # limitations under the License.
 from __future__ import absolute_import, division, print_function
 
-import unittest
-
 from transformers import is_torch_available
 
 from .test_configuration_common import ConfigTester
@@ -250,7 +248,3 @@ class DistilBertModelTest(CommonTestCases.CommonModelTester):
     #     for model_name in list(DISTILBERT_PRETRAINED_MODEL_ARCHIVE_MAP.keys())[:1]:
     #         model = DistilBertModel.from_pretrained(model_name, cache_dir=CACHE_DIR)
     #         self.assertIsNotNone(model)
-
-
-if __name__ == "__main__":
-    unittest.main()