User should not be able to run an unsupported test type for a particular task (ie: swap_entities for question-answering)

[luca-martial]

swap_entities is only valid when testing NER. As we are adding support for more and more tasks (question-answering, summarization), it becomes more and more important to add some kind of check to make sure the user does not try to run an unsupported test type on a certain task (like question-answering). The solution does not necessarily have to be a check, it could be something more efficient if you can think of a solution