Add InternalAffairs/UndefinedConfig cop.

[dvandersluis]

Adds an internal affairs cop to look for cops that use a configuration key without it being defined in config/default.yml.

I ran into this while adding a cop and accidentally putting a configuration in the wrong element. This internal affairs cop ensures that any configuration key used with cop_config in a cop is present in the configuration for that cop.

---

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.

[dvandersluis]

@koic the cop caught this -- in #8953, this configuration appears to have been accidentally added to the configuration of Lint/MultipleComparison instead of Style/MultipleComparison!

[koic]

Great! This looks like a useful cop. The only thing I'm concern about is that rubocop-minitest may have a false positive.
https://github.com/rubocop/rubocop-minitest

diff --git a/Gemfile b/Gemfile
index ad573e7..6d83faa 100644
--- a/Gemfile
+++ b/Gemfile
@@ -8,6 +8,6 @@ gemspec

 gem 'bump', require: false
 gem 'rake'
-gem 'rubocop', github: 'rubocop/rubocop'
+gem 'rubocop', path: '/path/to/rubocop/rubocop'
 gem 'rubocop-performance', '~> 1.11.0'
 gem 'yard', '~> 0.9'

% bundle exec rake
(snip)

Offenses:

lib/rubocop/cop/minitest/multiple_assertions.rb:58:36: C: InternalAffairs/UndefinedConfig: Max is not defined in the configuration for Minitest/MultipleAssertions in config/default.yml.
          Integer(cop_config.fetch('Max', 3))
                                   ^^^^^

88 files inspected, 1 offense detected

rubocop-minitest can make a workaround to disable this cop if it's not easy to solve.

[dvandersluis]

@koic should that cop not define Max: 3 in its configuration?

[koic]

Max: 3 looks like it's defined 🤔
https://github.com/rubocop/rubocop-minitest/blob/v0.15.0/config/default.yml#L119

[dvandersluis]

Oh strange, ok I'll look into it thanks!

[dvandersluis]

@koic I haven't been able to replicate this, I ran the cop on rubocop-minitest and no offenses were raised...

[koic]

@dvandersluis I checked again, but I couldn't reproduce the offense (maybe there was a problem with my local environment...?). Anyway, I'm sorry to take your time. And thank you again for implementing the useful cop!

[dvandersluis]

Thanks @koic!