Add integration tests for LightGBM

[parano]

Currently, LightGBM artifact code is not tested: https://codecov.io/gh/bentoml/BentoML/tree/master/bentoml/frameworks

We would want to add an integration test suit to test its functionalities including save, load, and run inference from API server. The setup should be similar to other integration tests that can be found here: https://github.com/bentoml/BentoML/tree/master/tests/integration

[yubozhao]

Closing issue. Thanks to @telescopic !