Change variable name test_data > validation_data (#2018)
Change variable name test_data > validation_data (#2018)

* it is confusing to name validation data `test_data` especially as terms like train, validation, test splits are common in ML. Change variable name in python quick start.
henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM