ENH: CXX tests should not fail if VTK_DATA_ROOT is missing.

The vtkTestingInteractor was trying to run regression tests even if
VTK_DATA_ROOT was not defined.

Many Testing/Cxx CMakeLists.txt files tried to run tests that required
input data, even though VTK_DATA_ROOT was not defined.

Subsequent topics will address Tcl and Python tests.

Change-Id: Iabf242f9f498ce044948ca1d4a29378b76ceb21d
Merge topic 'TestingWithoutData' into master

eb4be53c ENH: CXX tests should not fail if VTK_DATA_ROOT is missing.