Increase timeouts for VTKWriter tests [ci skip]

The VTK tests are close to the timeout on the CI system and sometimes
fail.

In addition the parallel vtktest has one process busy with polling in a
MPI function, taking 100% CPU for doing nothing, while the main process
is running the Python script. In case of parallel builds, this
unexpected resource usage slows down the test even more.

(cherry picked from commit dce08c0d)