As a highly interesting programming language is not supported and likely will not be supported in the next years I have following idea:
The user reads the puzzle description on your web page. The input / expected output values are available on a local programm written be me. The programm will then execute the tests locally on the computer.
The question is whether the input/output values are your Intellectual Property or you would allow this usage?
This way you are collecting the test cases. I see no problem as I believe it is covered by a very loose Creative Common license (https://creativecommons.org/licenses/by-sa/2.0/) which says you are free to copy and redistribute with just a little bit restrictions.