Fix pvtu output when VTK is build via CPM
Fixes https://gitlab.opengeosys.org/ufz-bgr/hpc/docs/-/issues/23.
Added a test for this (new CI job 'build mac petsc').
Also updated gtest for gcc 12. Two tolerances needed to be slightly relaxed for gcc 12 on envinf3.
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Feature description was added to the changelog -
Tests covering your feature were added?