luhn-number: provide CI example

[BNAndras]

In #195, it was discovered the current Luhn example does not pass the test CI. There are no community solutions to copy so we'll need to add a new example so the CI passes. @ErikSchierboom, can we tag this issue so it shows up on the https://exercism.org/contributing/tasks page as an exercise to improve? The exercise also uses custom tests which we should swap out for the upstream ones from public-specifications.

[kahgoh]

I'd be happy to look into fixing the Luhn exercise.

[ErikSchierboom]

I guess I don't have to tag it then :)