cmake: fix python build when BUILD_TESTING=OFF

Author: Mizux

ortools/python/setup.py.in

@@ -128,7 +128,7 @@ setup(
         '@PYTHON_PROJECT@.util.python':[
             '$<TARGET_FILE_NAME:sorted_interval_list_pybind11>',
             '$<TARGET_FILE_NAME:solve_interrupter_pybind11>',
-            '$<TARGET_FILE_NAME:solve_interrupter_testing_pybind11>',
+            $<$<TARGET_EXISTS:solve_interrupter_testing_pybind11>:'$<TARGET_FILE_NAME:solve_interrupter_testing_pybind11>',>
             '*.pyi'
         ],
     },