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)

That's a lot. Woohoo!

This can probably be reduced further by having the python process return 77 when importing the vtk module failed. Then we don't need the seperate python process that solely checks for that.

I should mention: this conflicts with !193 (merged), because both touch the subsamplingvtktest.

@joe: Indeed. Besides the merge conflict, !193 (merged) would also need to include the "intervals" in the filename. Using the same filename twice cannot work with running Python only at the end (and is detected as an error).

!193 (merged) is kind-of a new feature, so I'd want to give it a week or so between beeing ready and merging it. If you merge this one first, I'll fix up !193 (merged).

@joe: Okay, then I plan to merge this some time tomorrow.

mentioned in merge request !193 (merged)

Very cool, thank you @ansgar!

changed the description

assigned to @ansgar

merged

mentioned in commit e5f8ea19