gh-156570: handle lazy warnings import in test helper

[mikemikimike]

Issue: #156570

Summary

warnings_helper._filterwarnings() indexed sys.modules['warnings'] directly. Under -X lazy_imports=all, evaluating the module-level import can leave that entry unresolved, causing check_warnings() to raise KeyError.

Use the already imported module as a fallback when the warnings entry is absent, and add regression coverage for this state.

Testing

• python Lib/test/test_warnings_helper.py
• python -m py_compile Lib/test/support/warnings_helper.py Lib/test/test_warnings_helper.py
• git diff --check

Compatibility

The change preserves the existing module replacement behavior used by test_warnings while avoiding a KeyError when lazy imports have not populated sys.modules.

AI assistance was used to identify the issue and prepare this patch.

• Issue: test.support.warnings_helper.check_warnings() raises KeyError: 'warnings' under -X lazy_imports=all #156570

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If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.