Fix extruder assignment and tests

xs/xsp/Config.xsp

@@ -25,6 +25,7 @@
     std::vector<std::string> get_keys()
         %code{% THIS->keys(&RETVAL); %};
     void erase(t_config_option_key opt_key);
+    void normalize();
 };
 
 %name{Slic3r::Config::Print} class PrintConfig {