[R-package] added tests on LGBM_BoosterRollbackOneIter_R (#3006)
[R-package] added tests on LGBM_BoosterRollbackOneIter_R (#3006)


henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM