Enable stdout output in test mode, or add option for it

[Kimundi]

The test mode of playpen right now behaves just like "cargo test", in that it doesn't print to stdout per default, only in case of error.

However, seeing how playpen is presumably used more for quick tests and debugging, it would probably make sense to just disable that feature, or at the very least add an option for it.

See https://is.gd/mecpvH for an example of the current behavior.