Merged requested to merge fix/vtkreader-test into master
What this MR does / why does DuMux need it:
Fixes the vtkreader_1d_polyline test. The
test_vtk_staggeredfreeflowpvnames test deleted all pvd and vtu files in the folder, such that when executing the tests in paralell, The vtp file that is loaded by the polyline test didn't exist anymore.