[vtk-hdf] test more features

[dglaeser]

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Some of the features are currently not tested:

• cell data for vtk-hdf-image-data
• regression-testing of vtk-hdf transient files

Once vtk 9.2.7 is out, which should contain the respective fixes/features, we should add this to the test suite.

[dglaeser]

Edit: with #199 we modified the exit code computation in the regression test script. It used to take the max of all return codes, however, it seemed that when reading vtk-hdf image grid files, a return code of -6 was discarded with this and the test passed although an invalid pointer free operation occurs. This is due to a bug in the reader, which has already been fixed in VTK.

As with many other hdf-related features, we have to wait until the 9.2.7 release of VTK. Until now, all vtk-hdf-image grid regression tests are deactivated.