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.

dune/grid/io/file/test/CMakeLists.txt

@@ -13,11 +13,12 @@ dune_add_test(SOURCES nonconformboundaryvtktest.cc)
 dune_add_test(SOURCES printgridtest.cc)
 
 dune_add_test(NAME subsamplingvtktest
-              SOURCES subsamplingvtktest.cc test-linking.cc)
+              SOURCES subsamplingvtktest.cc test-linking.cc
+              TIMEOUT 600)
 
 dune_add_test(SOURCES vtktest.cc
               MPI_RANKS 1 2
-              TIMEOUT 600)
+              TIMEOUT 1200)
 
 dune_add_test(SOURCES vtksequencetest.cc)