TEST: Speed up GroupBy test setup

[vyasr]

Description

Reuse read-only GroupBy source data and coalesce cases that execute the same JIT-generated UDF so a pytest worker can reuse its process-local compilation cache. Each original dataset, dtype, special value, assertion, warning check, and expected-failure path still executes; tests copy inputs before mutation. The duplicate lambda x: x.nunique() spelling is removed because it exercises the same GroupBy reduction as the existing "nunique" case.

Checklist

• I am familiar with the CONTRIBUTING.md guidelines.
• New or existing tests cover these test-only changes.
• Documentation updates are not needed.

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📥 Commits

Reviewing files that changed from the base of the PR and between a8ad204 and 5ec0259.

📒 Files selected for processing (5)

• python/cudf/cudf/tests/groupby/test_apply.py
• python/cudf/cudf/tests/groupby/test_head_tail.py
• python/cudf/cudf/tests/groupby/test_nunique.py
• python/cudf/cudf/tests/groupby/test_sample.py
• python/cudf/cudf/tests/groupby/test_transform.py

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📝 Summary

Summary by CodeRabbit

• Tests
  • Expanded groupby test coverage for null keys and values, special values, correlations, and large datasets.
  • Improved comparisons between cuDF and pandas results for transform operations.
  • Streamlined reduction, uniqueness, sampling, and index-related test cases.
  • Improved test fixture reuse and documented read-only data expectations, reducing unnecessary setup and improving test efficiency.

Walkthrough

Changes

Groupby test updates

Layer / File(s)	Summary
Shared fixture lifecycle and data construction python/cudf/cudf/tests/groupby/test_apply.py, python/cudf/cudf/tests/groupby/test_head_tail.py, python/cudf/cudf/tests/groupby/test_sample.py, python/cudf/cudf/tests/groupby/test_transform.py	Fixtures now use module scope. Large datasets use CuPy tiling. Tests document read-only fixture use. Transform tests reuse a precomputed pandas reference.
Reduction and correlation coverage python/cudf/cudf/tests/groupby/test_apply.py	Unary reduction, index-reduction, and correlation tests reuse copied datasets and consolidate special-value, warning, and expected-failure handling.
Focused groupby assertions python/cudf/cudf/tests/groupby/test_nunique.py	The nunique test directly checks groupby(...).nunique() without aggregation-form parametrization.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: ⚪ Minimal · up to 5ec02

The test optimization preserves the existing GroupBy test behavior and coverage contracts, with no current merge-blocking risk identified.

Suggested reviewers: mroeschke, matt711, galipremsagar

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 13.64% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 22 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: speeding up GroupBy test setup.
Description check	✅ Passed	The description directly explains reuse of read-only test data and JIT caches while preserving test coverage and assertions.

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[bdice]

/merge