Fix tests on Julia v1.12

[JoshuaLampert]

See #57 (comment). I adapted this from trixi-framework/TrixiTest.jl#14.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.56%. Comparing base (1358e5d) to head (726dbc4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   92.82%   94.56%   +1.74%     
==========================================
  Files           5        5              
  Lines         237      313      +76     
==========================================
+ Hits          220      296      +76     
  Misses         17       17              

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