TestAnswers502WhenTheAppCannotBeReached closed a listener and pointed
smallwebwaf at its port, which another test running in parallel could
open in between, so the test sometimes got that server's answer instead
of a 502. It now uses 127.0.0.1:1: nothing listens there, and a test
listening on port 0 is always given a port from 32768 up, so no test
can take it. No other test reuses a closed listener's port.
Model: opus-5-5