test: cover HumanEval evaluator details

[motodriver]

Motivation

HumanEval and HumanEvalPlus expose per-sample details, but the evaluator output contract was not covered by regression tests. This is related to #723.

Changes

• mock HumanEval result files and verify per-sample is_correct, prompts, and pass@k scores
• mock EvalPlus results and verify base/plus statuses, composed solutions, and pass@1
• avoid executing generated code or downloading models in tests

Verification

• focused isolated unittest run: 8 passed
• python3 -m py_compile tests/datasets/test_humaneval.py
• git diff --check

The full local import path requires optional OpenCompass runtime dependencies that are not installed in this environment; CI will run the test in the project environment.