New cop: Minitest/PrivateTestMethod

[fatkodima]

While adding a new test case to a large codebase (rails/rails as an example) with its test files sometimes having thousands of lines of code and not following TDD (as I believe most of people do, mine included, except for bugfixes), it is very easy to add a test case to the private class section. The test case won't be run and the developer will think that tests are passing.

# bad
class FooTest < Minitest::Test
  def test_foo; end

  private
  def test_bar
  end
end

# good
class FooTest < Minitest::Test
  def test_foo; end
  def test_bar; end
end

# good
class FooTest < Minitest::Test
  def test_foo; end

  private
  def bar
  end
end

[koic]

I agree that this is probably out of the xUnit execution pattern. However, I'm not sure if that was intentional or a mistake by user. First of all, it may be better to specify it in Minitest Style Guide as it will become a stronger basis as a rule:
https://github.com/rubocop/minitest-style-guide

[andyw8]

I'd say that if the test name begins test_ then it's reasonable to assume it was a mistake to make it private.

[fatkodima]

I think, we could also check for assertions inside that method. In this case it is 100% a mistake.

[koic]

Okay, It doesn't have to be checked inside the method. Note, protected should also be treated the same as private. So, the cop name can be adjusted if we get good inspiration.