Follow minitest API, and use result_code to label the test result

We have some code that changes the `result_code` depending on a few new Assertion we have.
That works fine on raw minitest, as the minitest default reporter uses `result_code` ( https://github.com/seattlerb/minitest/blob/master/lib/minitest.rb#L454 ), however when adding a minitest-reporters reporter, it stopped showing the right result, as it is not using the method.