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_encoder_decoder.py

@@ -48,7 +48,3 @@ class EncoderDecoderModelTest(unittest.TestCase):
 
         with self.assertRaises(ValueError):
             _ = Model2Model.from_pretrained("does-not-exist")
-
-
-if __name__ == "__main__":
-    unittest.main()