Fix URL for download of sample test cases

It seems that the download URL for the sample test cases zip file depends on the host URL, not the particular problem URL.

I.e., if you `get` the problem at the URL `https://itu.kattis.com/courses/KSALDES1KU/KSALDES1KU-2023/assignments/nute7h/problems/pebblesolitaire2`, the zip file with test cases is not bound to that course URL, but rather to the "simple" problem URL: `https://itu.kattis.com/problems/pebblesolitaire2/file/statement/samples.zip`.