[R] More fixes for test n_threads.

[trivialfis]

close #12258

[Copilot]

Pull request overview

This PR updates the XGBoost R package test suite to reduce thread-related nondeterminism and address a CRAN pre-test failure (issue #12258), primarily by enforcing single-thread execution in more test cases.

Changes:

• Add nthread = 1 (or nthread = n_threads where n_threads is set to 1) to additional xgb.train() parameter lists and DMatrix constructors used in tests.
• Set data.table to single-thread mode in tests/testthat.R via data.table::setDTthreads(1) before running test_check().

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
R-package/tests/testthat/test_poisson_regression.R	Forces single-thread training parameters in Poisson regression tests to improve determinism.
R-package/tests/testthat/test_helpers.R	Ensures xgb.plot.tree test trains with nthread = 1 even when params are passed as a plain list.
R-package/tests/testthat/test_custom_objective.R	Pins nthread = 1 in multi-target custom metric tests to reduce thread-driven variance.
R-package/tests/testthat/test_basic.R	Adds/propagates nthread to more DMatrix creation and training params in several test cases.
R-package/tests/testthat.R	Sets OpenMP and data.table thread counts to 1 before executing the test suite.

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