Document guidelines on testing

I think we should come up with some guidelines on how analyzers should be tested. My proposal would be:

- Unit tests should exist to obtain as much code coverage as possible. Correctness of each exercise analyzer should be tested that way.
- Each exercise should have at least two smoke tests: one with an optimal solution that receives no feedback, and one with a solution that receives at least one exercise-specific comment.
- Next to that, a few smoke tests to cover exercises for which no analyzer is implemented should be present too, to make sure the analyzer works properly for every exercise. By that I mean that it shouldn't crash or something.

Once we come up with some concrete guidelines, we should probably write them down in the docs.

_Originally posted by @sanderploegsma in https://github.com/exercism/java-analyzer/issues/122#issuecomment-1924524027_