cmake: use 'Python3_' instead of PYTHON_' executable option

Only Python3_EXECUTABLE is set when using python, these tests weren't
compiled and used previously.

Parallel/Core/Testing/Cxx/CMakeLists.txt

@@ -6,7 +6,7 @@ vtk_add_test_cxx(vtkParallelCoreCxxTests tests
   )
 vtk_test_cxx_executable(vtkParallelCoreCxxTests tests)
 
-if (PYTHON_EXECUTABLE)
+if (Python3_EXECUTABLE)
   vtk_module_test_executable(
     vtkParallelCore-TestSocketCommunicator TestSocketCommunicator.cxx)
 

Rendering/Parallel/Testing/Cxx/CMakeLists.txt

@@ -17,7 +17,7 @@ set(all_tests
   )
 vtk_test_cxx_executable(vtkRenderingParallelCxxTests-MPI all_tests)
 
-if (PYTHON_EXECUTABLE)
+if (Python3_EXECUTABLE)
   vtk_module_test_executable(
     vtkRenderingParallel-TestClientServerRendering TestClientServerRendering.cxx)