testERP5TestNode: use mock to not isolate monkey patches

we don't want to patch spawn for all tests

erp5/tests/testERP5TestNode.py

@@ -578,7 +578,7 @@ shared = true
       call_parameter_list.append(args)
 
     test_node = self.getTestNode()
-    test_node.process_manager.spawn = spawn
+    with mock.patch.object(test_node.process_manager, 'spawn', side_effect=spawn):
       runner = test_type_registry[my_test_type](test_node)
       # Create and initialise/regenerate a nodetestsuite
       node_test_suite = test_node.getNodeTestSuite('foo')