[R-package] [tests] reduce verbosity in unit tests

[jameslamb]

Summary

Unit tests in the R package which are not explicitly testing {lightgbm}'s behavior for different values of the parameter verbose should use verbose = -1L to suppress all other log outputs.

Motivation

Suppressing more log output in tests may make them faster by reducing the number of times the package has to write to stdout / stderr.

It will also improve the usability of the logs for quickly identifying where issues happened.

Description

This work should be done in multiple small pull requests, not one huge one changing all tests.

It should be possible to override the value of verbose should be using an environment variable, so that developers can occasionally, easily run the tests in "debug" mode if additional logs are needed to identify issues.

Maybe like this:

LGB_TEST_VERBOSITY <- as.integer(Sys.getenv("LGB_TEST_VERBOSITY", "-1"))

test_that("some test", {
    params = list(verbose = LGB_TEST_VERBOSITY)
    ...
})

References

Created from this comment: #4567 (review)

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