Add to the Testing/Data directory ExternalData content links mirroring
the content and layout we use from the VTKData/Data directory.

Add a CMake/vtkLegacyData.cmake module, included from CMakeLists.txt and
Testing/External/CMakeLists.txt, to call ExternalData_Expand_Arguments
for all data directories added from VTKData/Data.  This will bulk-fetch
all test input data currently used by tests of any module, enabled or
not.  We can make the DATA{} references more granular later.

Add a VTK_TEST_DATA_DIR variable to refer to the directory we tell the
ExternalData module to populate the real data files.  Replace references
to VTK_DATA_ROOT in test command lines (after -D) with references to
VTK_TEST_DATA_DIR.  Drop the VTK_DATA_ROOT CMake option and stop using
the VTKData repository.

Remove the TESTING_DATA option from the vtk_add_test_cxx API and remove
the VTK_DATA_ROOT option from the vtk_add_test_mpi API since all test
input data are now handled t...