[R-package] silence more logs in tests

[jameslamb]

Contributes to #4862.

Silences additional logs produced by the R package's unit tests.

Notes for Reviewers

This PR reduces the number of logs printed by the R package's tests from 1616 lines to 636 lines.

sh build-cran-package --no-build-vignettes
R CMD INSTALL --with-keep.source ./lightgbm_3.3.2.99.tar.gz
cd R-package/tests
Rscript testthat.R > ../../out.log 2>&1
echo "log lines from tests: $(cat ../../out.log | wc -l)"

[StrikerRUS]

This PR reduces the number of logs printed by the R package's tests from 1616 lines to 636 lines.

That's great! 🚀 Thank you!

[github-actions]

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