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_model_card.py

@@ -80,7 +80,3 @@ class ModelCardTester(unittest.TestCase):
             model_card_second = ModelCard.from_pretrained(tmpdirname)
 
         self.assertEqual(model_card_second.to_dict(), model_card_first.to_dict())
-
-
-if __name__ == "__main__":
-    unittest.main()