fix benchmarks compatibility with newer pytest-cases

[jameslamb]

Description

Reverts changes from #14756.

• updates cudf's tests to be compatible with the latest pytest-cases (version 3.8.2)
• puts a floor of pytest-cases>=3.8.2 on that project to be sure older versions aren't used

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

Notes for Reviewers

The fix here was to stop using pytest-cases's automatic collection of cases, and to instead explicitly tell it where to look.

Per the pytest_cases.get_all_cases() docs (docs link)

cases:
a case function, a class containing cases, a module or a module name string (relative module names accepted).
...
When a module is listed, all of its functions matching the prefix, filter and has_tag are selected, including those functions nested in classes following naming pattern *Case*.

Since there are so few uses of pytest_cases in this project, I think explicitly passing module names is preferable to adjusting the repo to work with the new patterns introduced in smarie/python-pytest-cases#320.

For clarity, I'm also proposing renaming files that only contain test cases such that they begin with cases_*, but this isn't strictly necessary... happy to revert that if you'd like.

How I tested this

following CONTRIBUTING.md (click me)

Based on the advice in https://docs.rapids.ai/resources/reproducing-ci/, pointed build scripts at local output directories.

sed -ri '/rapids-download-conda-from-s3/ s/_CHANNEL=.*/_CHANNEL=${RAPIDS_CONDA_BLD_OUTPUT_DIR}/' ci/*.sh

Then re-generated the dependency files.

rapids-dependency-file-generator --clean

Then created a conda environment to run tests in and built the library.

conda env create \
  --name cudf-dev \
  --file ./conda/environments/all_cuda-120_arch-x86_64.yaml

source activate cudf-dev

./ci/build.sh libcudf cudf

Then ran the benchmarks, following

cudf/ci/test_python_cudf.sh

Line 30 in 726a7f3

# Run benchmarks with both cudf and pandas to ensure compatibility is maintained.

pushd python/cudf

rm -rf ./.pytest_cache

CUDF_BENCHMARKS_DEBUG_ONLY=ON \
pytest \
  --cache-clear \
  --numprocesses=16 \
  --dist=loadscope \
  --cov-config=.coveragerc \
  --cov=cudf \
  --cov-report=term \
  benchmarks

rm -rf ./.pytest_cache

CUDF_BENCHMARKS_USE_PANDAS=ON \
CUDF_BENCHMARKS_DEBUG_ONLY=ON \
pytest \
  --cache-clear \
  --numprocesses=8 \
  --dist=loadscope \
  --cov-config=.coveragerc \
  --cov=cudf \
  --cov-report=term \
  benchmarks

popd

Saw those tests succeed with my changes, and fail like #14712 (comment) without them.

I also tried adding a raise RuntimeError() to the test cases in e.g. cases_dataframe.py, just to convince myself that the benchmark cases were actually being successfully collected and run.

[mroeschke]

LGTM

[bdice]

/merge