Fix a false positive for Lint/DuplicateMethods

[koic]

This PR fixes the following false positive cases for Lint/DuplicateMethods when using duplicate nested method.

It prevents false positives in test cases such as:

def test_foo
  class A
    def x
      prepare_for_foo
    end
  end

  assert...
end

def test_bar
  class A
    def x
      prepare_for_bar
    end
  end

  assert...
end

The above is a false positive found in the rails/rails repository.

Also as an edge case, nested methods may have been intended.

def foo
  def x
    puts 'foo'
  end
end

def bar
  def x
    puts 'bar'
  end
end

foo
x #=> foo

bar
x #=> bar

foo
x #=> foo

---

Before submitting the PR make sure the following are checked:

• 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).
• Feature branch is up-to-date with master (if not - rebase it).
• 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.

[bbatsov]

Looks good. Thanks!