[Fix #279] Add new Minitest/NonExecutableTestMethod cop

[koic]

Resolves #279.

This PR adds new Minitest/NonExecutableTestMethod cop, which checks for the use of test methods outside of a test class. Test methods should be defined within a test class to ensure their execution.

NOTE: This cop assumes that classes whose superclass name includes the word "Test" are test classes, in order to prevent false positives.

# bad
class FooTest < Minitest::Test
end
def test_method_should_be_inside_test_class
end

# good
class FooTest < Minitest::Test
  def test_method_should_be_inside_test_class
  end
end

---

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.

[yahonda]

Thanks for implementing the Minitest/NonExecutableTestMethod cop. I have opened rails/rails#50379 to enable this cop at Rails.