[Fix #73] The Minitest department works on _test.rb by default

[koic]

Fixes #73.

This PR changes the Minitest department works on file names end with _test.rb by default.

RuboCop Minitest only targets files in test directory by default until v0.8.1.

Minitest:
  Enabled: true
  Include:
    - '**/test/**/*'

https://github.com/rubocop-hq/rubocop-minitest/blob/v0.8.1/config/default.yml

But test files may be located in a directory other than test directory.
This PR adds files that end of filename is _test.rb to be considered test files.

This behavior is similar to how RuboCop RSpec works on _spec.rb.
https://github.com/rubocop-hq/rubocop-rspec/blob/v1.38.1/config/default.yml#L5

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Before submitting the PR make sure the following are checked:

• 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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• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.