Documentation: Document all function in test headers properly

[lenaploetzke]

Describe your changes here:
Closes #2153
Closes #2152

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General

• The reviewer executed the new code features at least once and checked the results manually.
• The code follows the t8code coding guidelines.
• New source/header files are properly added to the CMake files.
• The code is well documented. In particular, all function declarations, structs/classes and their members have a proper doxygen documentation. Make sure to add a file documentation for each file!
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Tests

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License

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.95%. Comparing base (5952467) to head (32e7e0b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2172   +/-   ##
=======================================
  Coverage   77.95%   77.95%           
=======================================
  Files         113      113           
  Lines       19089    19089           
=======================================
  Hits        14881    14881           
  Misses       4208     4208           

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