Scala test fw

First attempt to use the inhouse Scala testing framework with one test for checking the picture overlay functionality. Keep in mind that I am a beginner Scala dev so code will likely look very "Javaish"
This `errors` will always be empty since it is a `val`. The `checkElementExistsAndCreateError` method will always return one error onto the `head` of an empty `List`.

There are a few ways you could get around this, for example using `folds` or `map/flatMap`s.
If you wanted to use `flatMap`, `checkElementExistsAndCreateError` could return an `Option[T]` and you could `flatMap` the method over the `List`, and then check if that new list is empty.