It would be nice to have a function, that returns an array of all variations for an observed PIN with a length of 1 to 8 digits

Where is the 5 to 8 digits part? ;-)

As always, not an issue unless you can point out why the tests might be wrong ;-)

Next time you should raise as a question and post your code with spoilers. You'll quickly get pointed out where your code has a bug ;-)