[tests][python] Add test for single leaf in linear tree (#4015)
[tests][python] Add test for single leaf in linear tree (#4015)

* Update test_engine.py

* Update python_package.yml

* Update python_package.yml

* Update test_engine.py

* hotfix
henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM