use parser.parse_graph for all test cases in test_onnx.py

[CharlesXin]

Use parser.parse_graph to generate all onnx graphs in test_onnx.py.

Example:
https://github.com/onnx/onnx/blob/08531476e70f23df402e11337349a97682f655bb/onnx/test/parser_test.py

Source Code Path:
python/test/test_onnx.py

To Test:
$ test_onnx.py

Bonus: 500pt