Build the VTK test data directory properly

If VTK is not the top level directory, an extra set of path components
will be injected between ExternalData and Testing. Compute this path and
add it to the expected output directory.

Change-Id: I13dc4b412f6a785f57fd99896b37475510a9833e

CMakeLists.txt

@@ -40,7 +40,8 @@ if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
 endif()
 
 # Test input data staging directory.
-set(VTK_TEST_DATA_DIR "${ExternalData_BINARY_ROOT}/Testing")
+file(RELATIVE_PATH vtk_reldir "${CMAKE_SOURCE_DIR}" "${CMAKE_CURRENT_SOURCE_DIR}")
+set(VTK_TEST_DATA_DIR "${ExternalData_BINARY_ROOT}/${vtk_reldir}/Testing")
 
 # Test input data directory.
 set(VTK_TEST_INPUT_DIR "${VTK_SOURCE_DIR}/Testing/Data")