[ci] prevent C API tests from leaving files behind (fixes #6361)

[jameslamb]

fixes #6361

I was testing whether #6361 could be closed by recent PRs, and realized that I'd missed one local file when listing the test files there... "preb.txt", created by the C API tests.

This does the following:

• ensures that file isn't left behind
• changes all uses of "preb" to "preds" (it always refers to predictions, so I think "preb" was just a typo)

Notes for Reviews

How I tested this

Ran all the tests locally (on my Mac, with all optional dependencies installed)

# Python and C API tests
sh build-python.sh install
pytest tests/

# R tests
sh build-cran-package.sh --no-build-vignettes
MAKEFLAGS="-j4" R CMD INSTALL --with-keep.source ./lightgbm*.tar.gz
cd R-package/tests
Rscript testthat.R

# C++ tests
rm -rf ./build
cmake -B build -S . -DBUILD_CPP_TEST=ON -DUSE_OPENMP=OFF
cmake --build build --target testlightgbm -j4
./testlightgbm

Checked that no files were left behind.

# look for local files (including those ignored by git)
git check-ignore -v -- *
git status --ignored