tests: use vtk-osmesa to run the 3D plotting offscreen

Author: elenafuengar

.github/workflows/manual_tests_CPU.yml

@@ -28,6 +28,8 @@ jobs:
     - name: pip install
       run:  |
         pip install wakis['notebook']
+        pip uninstall vtk -y
+        pip install --extra-index-url https://wheels.vtk.org vtk-osmesa
 
     - name: Print versions
       run: conda list

.github/workflows/pull_request_CPU.yml

@@ -32,6 +32,8 @@ jobs:
         run: |
           cd wakis
           pip install .['notebook']
+          pip uninstall vtk -y
+          pip install --extra-index-url https://wheels.vtk.org vtk-osmesa
 
       - name: Print installed packages
         run: conda list