[R-package] limit number of threads used in tests and examples (fixes #5987)

[jameslamb]

Fixes #5987.

This PR fixes the two issues that blocked the release of {lightgbm} v4.0.0 to CRAN a few days ago, documented in #5987.

• ensures that at most 2 threads are used in all examples and most tests
• modifies 2 internal functions with .-separated names to instead use _

Notes for Reviewers

Once this is approved, I'll push a release containing this PR to CRAN, as v4.0.0.1 (since a new submission requires a new release number, and to avoid creating a new actual LightGBM release of all other components).

Why is this touching so many lines?

I tried many variations of the idea from #5102 (comment), using a global option (specified in R-package/tests/testthat.R or at the tops of test files) to pass through a number of threads set in Booster$new(). Unfortunately, they all resulted in segmentation faults (😬) from testthat. Maybe related to some interplay between the fact that {testthat} runs tests in sub-processes and the fact that session options are initialized when packages are loaded?

• https://stackoverflow.com/a/12599875/3986677
• https://testthat.r-lib.org/articles/parallel.html

I'm not sure. For now, to try to get a release up before the CRAN summer break, I think we should just hard-code num_threads to 2 in as many places as possible... can work out a more elegant solution later, when there isn't time pressure.

Other references I consulted while trying to find a better solution

• https://r-pkgs.org/testing-design.html
• Set option before pkg is loaded r-lib/testthat#1702
• https://stackoverflow.com/questions/12598242/global-variables-in-packages-in-r
• https://cran.r-project.org/web/packages/testthat/vignettes/special-files.html

[guolinke]

We will revert this PR when the "global option" works?

[jameslamb]

The parts of it about limiting number of threads in tests and examples, yes.

The renaming of those two internal functions should be permanent, now that CRAN is complaining about them.

[jameslamb]

Thanks @shiyu1994 ! I'll merge this and submit a new version to CRAN right now. Will @ you here when I do.

[jameslamb]

Ok @shiyu1994 I just submitted on CRAN as v4.0.0.1. Please check your inbox. Thanks for your help!

[github-actions]

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