[R-package] fix learning-to-rank tests on Solaris (#3534)
[R-package] fix learning-to-rank tests on Solaris (#3534)

* [R-package] fix learning-to-rank tests on Solaris

* Update R-package/tests/testthat/test_learning_to_rank.R

Co-authored-by: Nikita Titov <31eaea17affb14e78210388fc0bace3c2fd52ddc@mail.ru>

Co-authored-by: Nikita Titov <31eaea17affb14e78210388fc0bace3c2fd52ddc@mail.ru>
microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM