[Fix #10760] Fix a false positive for Lint/NonAtomicFileOperation

[koic]

Fixes #10760.

This PR fixes a false positive for Lint/NonAtomicFileOperation when using FileTest.exist? with if condition that has else branch.

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[ydah]

Thank you so much!
I also checked the following similar false positive test case and it was resolved. Looks good to me.

if FileTest.exist?(path)
  do_something
elsif foo
  FileUtils.makedirs(path)
end

unless FileTest.exist?
  FileUtils.makedirs(path)
else
  do_something
end

[bbatsov]

Looks good. Thanks!