Cleanup GPU histogram tests.

[trivialfis]

Extracted from #12630

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Advanced

Run ID: c1fb8e81-8d3a-479f-83a9-f54d3b3286ab

📥 Commits

Reviewing files that changed from the base of the PR and between 889c4d2 and 036c453.

📒 Files selected for processing (5)

• tests/cpp/histogram_helpers.cu
• tests/cpp/histogram_helpers.h
• tests/cpp/test_histogram_helpers.cu
• tests/cpp/tree/gpu_hist/test_histogram.cu
• tests/cpp/tree/gpu_hist/test_multi_histogram.cu

💤 Files with no reviewable changes (3)

• tests/cpp/test_histogram_helpers.cu
• tests/cpp/histogram_helpers.cu
• tests/cpp/tree/gpu_hist/test_multi_histogram.cu

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Walkthrough

The change updates ELLPACK test support and removes the MakeEllpackForTest helper and its test. GPU histogram tests add deterministic input generation and compare built device histograms with independently accumulated expected values. The tests also add parameterized layout and memory cases, large-row and wide-feature coverage, and shared-memory budget checks. Existing subtraction and categorical-gradient tests change, and the separate multi-histogram test suites are removed.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to 036c4

No actionable issue is established in the CUDA histogram test changes. The PR is ready to merge after normal checks.

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