Raise an error if external data store is needed
Historically (before VTK 9), VTK_BUILD_TESTING was on by default, and when VTK adopted automatic downloading of testing data (around 2013), a check was added to silently turn off testing when building from a VTK release tarball. The logic, according to bf5cf3cf, was to avoid situations where people would build from a tarball and have their computer attempt to download tons of data over the internet.
Since VTK 9.0, VTK_BUILD_TESTING defaults to OFF, so any justification for automatically turning it off is gone.
Instead, if VTK_BUILD_TESTING has been turned on, but there is no default location for downloaded data, raise FATAL_ERROR and ask for an external data directory to be specified.
For reference, see test-not-found on Discourse.
Edited by David Gobbi