Update test

Author: jorgensd

.github/workflows/main-test.yml

@@ -56,6 +56,8 @@ jobs:
           jupyter-nbconvert --to python fundamentals_code.ipynb
           python3 -c "from pyvista import start_xvfb; start_xvfb(0.1)"
           mpirun -n 2 python3 fundamentals_code.py
+          jupyter-nbconvert --to python nitsche.ipynb
+          mpirun -n 2 python3 nitsche.py
           jupyter-nbconvert --to python membrane_code.ipynb
           mpirun -n 2 python3 membrane_code.py
           cd ../chapter2