Add docs for AS::TestCase::test_order

Document `test_order` and `test_order=` from `ActiveSupport::TestCase`.

[ci skip]

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The reason why I'm opening this PR on [rails/rails](https://github.com/rails/rails) rather than pushing to [rails/docrails](https://github.com/rails/docrails) is because I wanted some feedback.

- Do you think it's a good idea to specify the possible values for `test_order`?
- Do you think the documentation should explain what these values mean?
- Or do you think there should be a link to [Minitest API doc](http://docs.seattlerb.org/minitest) instead?
Could this become `cattr_accessor :test_order # :nodoc:` ?