Fix preconditioner unit test

[codecov[bot]]

Codecov Report

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

Project coverage is 88.28%. Comparing base (a57d43b) to head (56e276a). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #283 +/- ## ========================================== + Coverage 88.25% 88.28% +0.03% ========================================== Files 7 7 Lines 1132 1127 -5 ========================================== - Hits 999 995 -4 + Misses 133 132 -1 ```

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[tmigot]

Thanks @mertkipcak and @mpf !