Add unit tests for checkers

[quantum5]

We should add some unit tests to ensure all checkers are working as expected in dmoj/tests, not just standard.

Note: A simple unit test has way less overhead than creating a problem to test it with in dmoj-testsuite, so we should prefer to use unit tests.

[kiritofeng]

Supersedes https://github.com/DMOJ/judge-testsuite/issues/24.

[Riolku]

Is this finished? I recall seeing unit tests.