Judge unit test refactor

[Xyene]

• [x] Since we're including a tests dir already, and the testsuite runner has been part of this repository since the beginning, we sho[uld make the testsuite a submodule too (preferably under the tests dir)
• [ ] Expose a --test flag to the dmoj entry point, which simply runs the tests (this would be a simple "is DMOJ borked on my system" test users could try out)