Improvement: Increase Codecoverage of the Fortran-Interface

[Davknapp]

Closes #1816

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[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.28%. Comparing base (5952467) to head (72c1e7c).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
.../t8_fortran_interface/t8_fortran_interface_mod.f90	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1815      +/-   ##
==========================================
+ Coverage   77.95%   78.28%   +0.32%     
==========================================
  Files         113      115       +2     
  Lines       19089    19105      +16     
==========================================
+ Hits        14881    14956      +75     
+ Misses       4208     4149      -59     

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