VTK test data can be useful to test ParaView data flows and code as a reference of known good data. The VTK file I/O documentation itself recommends using the example data files to test one’s own project software. A straightforward way to obtain the data is by “building” a single ExternalData target, where virtually all of the time is spent downloading.
git clone --recursive https://gitlab.kitware.com/vtk/vtk.git --depth 1
cmake -Bbuild -DVTK_BUILD_TESTING=ON
cmake --build build -t VTKData
Numerous data folders are created under build/ExternalData/Testing/Data. For example, VTKHDF data in HDF5 file is the file mandelbrot-vti.hdf