[R-package] add assertions to test on lightgbm() weights

[jameslamb]

I noticed the following today in the logs for the R package's tests.

══ Skipped ════════════════════════════════════════════════════════════════════════════════════
1. lightgbm() accepts 'weight' and 'weights' (test_basic.R:2931:1) - Reason: empty test

This is coming from a test added recently (#4975). I guess that {testthat} skips tests that don't have any expect_* calls in them. It looks like this has been the behavior of that package since 2016 (r-lib/testthat#413), I just never noticed it before.

This PR adds assertions to the relevant test, to ensure that it's run.

Notes for Reviewers

You can check the MSVC R-package CI jobs to see the logs from tests, since for those the CI runs Rscript testthat.R instead of R CMD check.

[jmoralez]

Nice catch! I see the following in the logs for this PR:

...................

and for #5109 I see the skipped one. (logs)

.................S

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.