GitLab

* Handling NULL objective when booster is printed, moving test functions out of method tests, creating unique tests for different method situations

* Correcting whitespace

* Adding types for integer slices

* Moving test functions nearer to relevant tests, setting print to be default

* Update R-package/tests/testthat/test_lgb.Booster.R

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Co-authored-by: James Lamb <jaylamb20@gmail.com>
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>