Add a Testing/Data directory with ExternalData content links mirroring
the content and layout we use from the VTKLargeData/Data directory.
Add a VTK_USE_LARGE_DATA option to enable tests using such data.
Call ExternalData_Expand_Arguments in */*/Testing/Cxx/CMakeLists.txt for
tests that reference these data files in order to tell the ExternalData
module to populate a ExternalData/Testing/Data directory in the build
tree. Pass the ExternalData/Testing directory in the build tree as the
"-D" data directory option to tests using these data so that they can
find the files where ExternalData puts them at build time.
In the vtk_add_test_* API replace the LARGE_DATA options with a new
TESTING_DATA option to indicate that a test uses the directory
ExternalData/Testing/Data populated by ExternalData from the
Testing/Data source directory.
Drop VTK_LARGE_DATA_ROOT and stop using the VTKLargeData repository.
Drop references to VTKLargeData from test source comments.