VTK tests: check all VTK files with one python invocation

This merge request replaces the checkVTKFile function with a VTKChecker class which allows to queue up files and then check all of them within a single python invocation. As importing the vtk Python module has a significant overhead, this reduces the time to run the tests noticeably:

Test	Old	New
vtktest	384 sec	21 sec
vtktest-mpi-2	945 sec	17 sec
subsamplingvtktest	198 sec	22 sec

for one of multiple parallel builds on the CI system.

Note that in the case of the parallel test using MPI, the second rank just busy-waits. If the test runs much quicker, the second process will only run a shorter time as well, removing CPU contention between parallel builds. This should explain the much larger effect of this change for the parallel case.

Closes: #67 (closed)