GitLab

* [R-package] fix segfaults caused by missing Booster and Dataset handles (fixes #4208)

* fix test errors

* fixes for cpplint

* Update R-package/tests/testthat/test_dataset.R
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* fix tests

* Apply suggestions from code review
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* move asserts inside try-catch
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>