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]

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

@@           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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