Fix: Fix a variable which caused the vtk test not to run

[sandro-elsweijer]

Closes #2160

Describe your changes here:
The vtk tests were build if T8_ENABLE_VTK is true, but the name of the variable is T8CODE_ENABLE_VTK

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.95%. Comparing base (d7259da) to head (ceb99f4).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2154   +/-   ##
=======================================
  Coverage   77.95%   77.95%           
=======================================
  Files         113      113           
  Lines       19087    19087           
=======================================
  Hits        14880    14880           
  Misses       4207     4207           

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[lenaploetzke]

Test is running now :)