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_common.py

@@ -378,7 +378,3 @@ def ids_tensor(shape, vocab_size, rng=None, name=None, dtype=None):
     output = tf.constant(values, shape=shape, dtype=dtype if dtype is not None else tf.int32)
 
     return output
-
-
-if __name__ == "__main__":
-    unittest.main()