Capture user output

[ErikSchierboom]

Each test case in the results.json file has the option of having an output field. This field is used when students are using the in-browser editor to help them with "debugging" their code (as they won't have an actual debugger available in the browser).
See the spec for more information.

You may either capture content that is output through normal means (e.g. puts in Ruby, print in Python or Debug.WriteLine in C#), or you may provide a method that the user may use (e.g. the Ruby Test Runner provides a user with a globally available debug method that they can use, which has the same characteristics as the standard puts method.

There is existing code to write the field (albeit with a default value) in https://github.com/exercism/scala-test-runner/blob/main/src/main/scala/Application.scala#L62