PidExecutor - an executor that waits for the specified file to exist

This is an executor that starts a process, then waits for a specified file to come into existance. Usefull for integration tests, etc.
If a different program were to create the specified file just after the given subprocess was started, then everything would appear to be fine. In the case of a pid file it's quite easy to check what program created the file (simply compare the subprocesses pid with the pid from the file), Of course psutil.Process(pid).get_open_files() would be perfect for this, as long as the subprocess doesn't close the file. This approach would be best for things like log files.