improve unit test coverage

[razumau]

We more or less properly test only two recent additions (roster continuity rules and missing rosters adjustments). There are some unit tests for utility functions in test_tools, but not even all those functions are covered.

We have end-to-end tests (calculate a release, verify that numbers haven’t changed), but they cannot cover all potential data issues. We can expand those tests as well, but it’s not feasible to calculate more than two or three releases in general CI runs (this would take too long).

Some functions probably need refactoring to make them testable.