vtk_common: enable testing by default

vtk_common.cmake

@@ -356,6 +356,7 @@ SITE:STRING=${CTEST_SITE}
 BUILDNAME:STRING=${CTEST_BUILD_NAME}
 CTEST_USE_LAUNCHERS:BOOL=${CTEST_USE_LAUNCHERS}
 DART_TESTING_TIMEOUT:STRING=${CTEST_TEST_TIMEOUT}
+VTK_BUILD_TESTING:STRING=WANT
 ${data_options}
 ${cache_build_type}
 ${cache_make_program}