Add test for anonymous module with private method

[eugeneius]

This already passes on RuboCop AST 1.0.1 (which is required since #8928) via rubocop/rubocop-ast#142, but I think it's worth adding as an integration test anyway, and we can also now drop the refinement-specific check added in #8653.

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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[koic]

Thanks!