Add new cop Minitest/RefuteNil

[tejasbubane]

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[tejasbubane]

I created a separate cop for now but should we have separate refute cops or have single cop which handles refute as well? This will apply to almost all other cops.

[koic]

Ah, that's a good point of view. RuboCop Minitest can be designed to provide a single cop that bundles refute_*.

However, a design that provides individual cops along The Minitest Style Guide may be fine. So, Minitest is a simple API and expects a limited number of style rules.

I think that it may be simple if a rule of Minitest Style Guide and a cop is mapped. Each cop can have a reference to a rule of The Minitest Style Guide.

Cc @rubocop-hq/rubocop-core

[koic]

Can you add ### New features category?

[tejasbubane]

added

[koic]

Please add this cop setting to config/default.yml.

[koic]

Thanks!

[koic]

I overlooked the version and updated it by b9c8da6. FYI, Minor version will be bumped when new cop is added.