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_tf_roberta.py

@@ -244,7 +244,3 @@ class TFRobertaModelIntegrationTest(unittest.TestCase):
         self.assertEqual(list(output.numpy().shape), expected_shape)
         expected_tensor = tf.constant([[-0.9469, 0.3913, 0.5118]])
         self.assertTrue(numpy.allclose(output.numpy(), expected_tensor.numpy(), atol=1e-3))
-
-
-if __name__ == "__main__":
-    unittest.main()