Refactor test generic

Folowing #66 (closed)

test_generic

• Try to import an onnx with a custom operator, (no assert are done)
• Export back the ONNX and assert that the generated ONNX is valid
• TODO: Use ONNX lib to assert that producers are well converted to initializers
• TODO: Use ONNX lib to assert that atributes are well converted
• TODO: Refactor this by avoiding saving ONNX and keeping it at ModelProto level