Fix Treelib test

[daico007]

PR Summary:

Fix test that checked for the output of show_hierarchy. Relate to #1141

PR Checklist

---

• [ ] Includes appropriate unit test(s)
• [ ] Appropriate docstring(s) are added/updated
• [ ] Code is (approximately) PEP8 compliant
• [ ] Issue(s) raised/addressed?

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c5872e7) 87.12% compared to head (7eec5ee) 87.12%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1142 +/- ## ======================================= Coverage 87.12% 87.12% ======================================= Files 62 62 Lines 6455 6455 ======================================= Hits 5624 5624 Misses 831 831 ```

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