[test]fix the exception that's raised by proxy test

the former test should be executed under `c` disk by default. otherwise, the `.reg` file that's generated under the root of `c` disk will not be deleted after the test, and then an exception will be threw about `fail to unlink file`. it's a mistake about the path `/`, which means the root path of the current path. but not `c`. I've revised it to `c` explicitly.