[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

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Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [x] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [x] 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 https://github.com/rails/rails/pull/50379 to enable this cop at Rails.