fix test__open_path_and_check_proc API test

> assert open_pid.isdigit(), open_pid
E AssertionError: 49557
E
E assert False
E + where False = <built-in method isdigit of str object at 0x8063cc9f0>()
E + where <built-in method isdigit of str object at 0x8063cc9f0> = '49557\n'.isdigit

The error message is absolute garbage and incomprehendable but once you stare at it long enough you'll realize it's broken because `open_pid` has a `\n` character at the end....