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_tokenization_distilbert.py

@@ -14,8 +14,6 @@
 # limitations under the License.
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import unittest
-
 from transformers.tokenization_distilbert import DistilBertTokenizer
 
 from .test_tokenization_bert import BertTokenizationTest
@@ -43,7 +41,3 @@ class DistilBertTokenizationTest(BertTokenizationTest):
         assert encoded_pair == [tokenizer.cls_token_id] + text + [tokenizer.sep_token_id] + text_2 + [
             tokenizer.sep_token_id
         ]
-
-
-if __name__ == "__main__":
-    unittest.main()