cmake: allow testing when forbidding downloads

The testing data might be available locally through the data tarballs
made with releases.

This mostly reverts !5079 but still makes noise in the situation since
it can be easy to forget it.

Fixes: #18692

CMakeLists.txt

@@ -267,10 +267,10 @@ mark_as_advanced(VTK_TARGET_SPECIFIC_COMPONENTS)
 set(_vtk_module_group_default_StandAlone "WANT")
 set(_vtk_module_group_default_Rendering "WANT")
 
-set(vtk_enable_tests "${VTK_BUILD_TESTING}")
-if (VTK_FORBID_DOWNLOADS)
-  message(STATUS "Module testing disabled due to `VTK_FORBID_DOWNLOADS` being set.")
-  set(vtk_enable_tests "OFF")
+if (VTK_FORBID_DOWNLOADS AND VTK_BUILD_TESTING)
+  message(STATUS
+    "Module testing will fail due to `VTK_FORBID_DOWNLOADS` without a local "
+    "copy of the testing data.")
 endif ()
 
 include(vtkModule)
@@ -336,7 +336,7 @@ vtk_module_scan(
   REQUIRES_MODULES    vtk_required_modules
   UNRECOGNIZED_MODULES vtk_unrecognized_modules
   WANT_BY_DEFAULT     "${VTK_BUILD_ALL_MODULES}"
-  ENABLE_TESTS        "${vtk_enable_tests}")
+  ENABLE_TESTS        "${VTK_BUILD_TESTING}")
 
 if (vtk_required_modules OR vtk_unrecognized_modules)
   message(FATAL_ERROR