Add RSpecRails/PendingOnlyExampleGroup cop

[r7kamura]

rspec-rails generates this kind of test file skelton,

require 'rails_helper'

RSpec.describe Post do
  pending "add some examples to (or delete) #{__FILE__}"
end

but it is not a good idea to git-add these files without any modification. So I propose to add a cop to suppress it.

In fact, many of the projects I've participated in have included a lot of meaningless pending test files like this, and RSpec was outputting a large amount of logs about them there.

---

Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded: "<<next>>" in default/config.yml.

If you have modified an existing cop's configuration options:

• Set VersionChanged: "<<next>>" in config/default.yml.

[bquorning]

I think most of what you want is covered by rubocop-rspec’s Pending cop. Except, that cop will also flag your last test example:

        RSpec.describe Post do
          pending "TODO"

          it 'does something' do
            expect(foo).to eq(1)
          end
        end

[r7kamura]

As you also pointed out, that would make legitimate pending and skip an offense. I think nothing wrong with marking existing examples as pending or skip for some reason.

[bquorning]

I think a configuration option to RSpec/Pending would be better than a new cop.

[r7kamura]

I believe that this kind of code is usually not written by humans and would only be generated by rspec-rails. That's why I proposed this cop to rubocop-rspec_rails. But I also agree that it should be proposed as an option for RSpec/Pending, since looking at the cop logic alone, there is no special Rails involvement 🤔