VTK_BUILD_TESTING 3 states are insufficient

Current VTK_BUILD_TESTING has the following three states:

'ON' - try to enable all modules that are part of TEST_DEPENDS of any module that is default. This means that on machines without MPI enabled the configuration errors out since 'VTK::ParallelMPI' is not enabled.
'OFF' - this works as expected
'DEFAULT' - this means that only modules that are currently enabled will be enabled. So for example if you enable VTK::AcceleratorsVTKm you will NOT get any tests unless you manually enable all of the following modules:

  VTK::CommonSystem
  VTK::FiltersSources
  VTK::IOImage
  VTK::IOLegacy
  VTK::IOPLY
  VTK::IOXML
  VTK::ImagingHybrid
  VTK::ImagingSources
  VTK::InteractionStyle
  VTK::RenderingFreeType
  VTK::RenderingOpenGL2
  VTK::RenderingVolumeOpenGL2
  VTK::TestingCore
  VTK::TestingRendering

If these modules are marked as DEFAULT they will not be enabled you have to force them to YES or WANT.

So what we need is the ability for a module to mark all TEST_DEPENDS as WANT so they become enabled if the system supports them.