tests/docs: allow local development on port other than 5000

[GraemeWatt]

For local development, the web app is deployed at http://localhost:5000. It might be useful to explain in the docs how to deploy the web app at a port other than 5000 by setting the Flask environment variable FLASK_RUN_PORT. However, some of the tests would also need to be modified to avoid hard-coding the URL http://localhost:5000 and instead get the port from the FLASK_RUN_PORT if set, otherwise use a default of 5000.

This issue arose because the new Airplay Receiver functionality on macOS Monterey means that Control Center listens on port 5000 by default and hence needs to be turned off before starting the HEPData web app at http://localhost:5000.