Add specs for nested conditionals in Style/UnlessElse cop

[tejasbubane]

Wrote these specs while debugging #3263. These specs already pass and do not fix the issue in any way. I felt these specs should be included, hence the PR. Feel free to close if you think it does not make much of a difference.

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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.
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  and description in grammatically correct, complete sentences.