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.

examples/summarization/test_utils_summarization.py

@@ -98,7 +98,3 @@ class SummarizationDataProcessingTest(unittest.TestCase):
 
         result = compute_token_type_ids(batch, separator)
         np.testing.assert_array_equal(result, expected)
-
-
-if __name__ == "__main__":
-    unittest.main()