test: Add tests for Monoid

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.36%. Comparing base (d90ee43) to head (90d035b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #254 +/- ## ========================================== + Coverage 66.17% 66.36% +0.18% ========================================== Files 110 110 Lines 11172 11178 +6 Branches 623 623 ========================================== + Hits 7393 7418 +25 + Misses 3779 3760 -19 ```

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