Make test_vtk_staggeredfreeflowpvnames test less aggressive
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.
Edited by Timo Koch