Add new cop Minitest/NoTestCases

[tejasbubane]

Closes #216

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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.

[fatkodima]

Lgtm 👍

[koic]

I tried this cop with the rails/rails repo and it seems to find a lot of false positives. Could you take a look?

[tejasbubane]

@koic Rails test suite defines lots of temp test classes without any tests. Some examples:

• https://github.com/rails/rails/blob/b8e09fc54ec369574ff34734514600428dbd8a2a/activerecord/test/cases/validations/uniqueness_validation_test.rb#L49-L60
• https://github.com/rails/rails/blob/b8e09fc54ec369574ff34734514600428dbd8a2a/railties/test/test_unit/reporter_test.rb#L8-L10
• https://github.com/rails/rails/blob/b8e09fc54ec369574ff34734514600428dbd8a2a/activesupport/test/cache/stores/redis_cache_store_test.rb#L119-L140

I am not sure how to fix them safely. Any ideas?

[koic]

Isn't this rule too strict? I'm concerned that there are more opportunities for false positives than for detected true offenses.　TBH, I don't have a sense of user-friendliness.

[fatkodima]

Rails test suite defines lots of temp test classes without any tests. Some examples:

https://github.com/rails/rails/blob/b8e09fc54ec369574ff34734514600428dbd8a2a/activerecord/test/cases/validations/uniqueness_validation_test.rb#L49-L60
https://github.com/rails/rails/blob/b8e09fc54ec369574ff34734514600428dbd8a2a/railties/test/test_unit/reporter_test.rb#L8-L10
https://github.com/rails/rails/blob/b8e09fc54ec369574ff34734514600428dbd8a2a/activesupport/test/cache/stores/redis_cache_store_test.rb#L119-L140
I am not sure how to fix them safely. Any ideas?

First example: it just incorrectly detects non test class. That should be fixed.
Second example: Some empty test class to test the reporter. Regular projects do not extend test runners and do not have such classes.
Third example: This is a definition of a base test class - a popular practice and in this case the cop should be just disabled inline, in my opinion.

Personally, I see a value of this cop. Maybe not inside the rails itself, where "people know what they are doing"™️ and it would just generate false positives (I had not tested on it, probably there would be many valid offenses). But for regular projects this will produce many valid offenses. We can make it disabled by default, for example.

[koic]

We can make it disabled by default, for example.

Yeah, it looks a reasonable idea. @tejasbubane Can you disable this cop by default?

[tejasbubane]

@koic Disabled by default.