Re-enabled parallel XML reader/writer test

This test was not being executed.

IO/ParallelXML/Testing/Python/CMakeLists.txt

-if(TARGET pvtkpython)
-  add_test(NAME ${vtk-module}Cxx-testParallelXMLWriters
-    COMMAND ${MPIEXEC} ${MPIEXEC_NUMPROC_FLAG} 4 ${MPIEXEC_PREFLAGS}
-      $<TARGET_FILE:pvtkpython> ${CMAKE_CURRENT_SOURCE_DIR}/testParallelXMLWriters.py
-      ${VTK_MPI_POSTFLAGS})
-endif()
+vtk_add_test_python_mpi(
+    testParallelXMLWriters.py,NO_VALID
+)