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_common.py

@@ -892,7 +892,3 @@ class ModelUtilsTest(unittest.TestCase):
             self.assertEqual(model.config.output_attentions, True)
             self.assertEqual(model.config.output_hidden_states, True)
             self.assertEqual(model.config, config)
-
-
-if __name__ == "__main__":
-    unittest.main()