tester.py not executable as given in readme

[AAndersn]

The new GridAPPSD-Python readme (https://github.com/GRIDAPPSD/gridappsd-python/blob/poetry/README.md) includes a tester.py script with a generic username and password. The user does not exist in the platform and so the tester.py script fails with the following message:

{'content-type': 'text/plain', 'message': 'Subject [app_user] is not authorized to write to destination: queue://send.topic'}
org.apache.shiro.authz.UnauthorizedException: Subject [app_user] is not authorized to write to destination: queue://send.topic

The readme probably should be updated to use a valid username / password combination or specify that this is a generic example and will not test if GridAPPSD-Python installed correctly.