Cover codebase with tests

[KvanTTT]

It would be great to have tests for different algorithms. It allows regression testing and it helps with familiarization.

[Mhmonicox]

It would be great to have tests for different algorithms.

[turf00]

Is there a reason why the project appears to have zero tests? Would you be open to PRs with tests added?

[amoeller]

PRs with tests are very welcome!