Update ExternalProjectDependency system based on commontk/Artichoke@cfefc7ea27

Remove test since they are now part of the Artichoke project.

See #418
Merge branch '418-support-use-system-option'

* 418-support-use-system-option:
  Update ExternalProjectDependency system based on commontk/Artichoke@cfefc7ea27
Update ExternalProjectDependency system based on commontk/Artichoke@70ed903

$ git shortlog cfefc7ea27..70ed903
Jean-Christophe Fillion-Robin (6):
      Properly exclude optional dependency when not only associated with top-level
      Remove unused test files
      Fix CMakeLists in MultiCall test to work as expected in the inner build case
      Properly set or restore the project variable. Fixes #4
      Make sure optional external project are not added.
      Rename "superbuild_message" to match ExternalProject naming convention
Merge branch '418-support-use-system-option'

* 418-support-use-system-option:
  Update ExternalProjectDependency system based on commontk/Artichoke@70ed903
Fix CTK build when not all plugins/apps/libraries are enabled.

Following commit 95f4262 teaching the build system how to understand
the USE_SYSTEM option, the enabling variables were systematically set
in the ctkBlocCheckDependencies module.

Since the logic deciding if "find_package(PackageName)" should be called
was simply done by just checking if the
"< <PackageName>_enabling_variable>_FIND_PACKAGE_CMD" variable was set
or not, the build was failing when done without all external project
required.

See #418
Merge branch '418-support-use-system-option'

* 418-support-use-system-option:
  Fix CTK build when not all plugins/apps/libraries are enabled.
Unconditionally rebuild "CTK-build" inner project. Restore commit 1be215aef3

See #32
COMP: Support CTK build using VTK6