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_auto.py

@@ -100,7 +100,3 @@ class AutoModelTest(unittest.TestCase):
         logging.basicConfig(level=logging.INFO)
         model = AutoModelWithLMHead.from_pretrained(SMALL_MODEL_IDENTIFIER)
         self.assertIsInstance(model, BertForMaskedLM)
-
-
-if __name__ == "__main__":
-    unittest.main()