test: use dynamic port allocation to avoid test conflicts

Use port=0 for dynamic port allocation in test framework components
to avoid "address already in use" errors from concurrent tests or
ports stuck in TIME_WAIT state from previous test runs.

Changes:
- socks5.py: Update conf.addr after bind() to reflect actual port
- p2p.py: Retrieve actual port after create_server() when port=0
- feature_proxy.py: Use port=0 for all SOCKS5 proxy servers
- feature_anchors.py: Use port=0 for onion proxy server

test/functional/test_framework/p2p.py

@@ -787,13 +787,14 @@ class NetworkThread(threading.Thread):
                 cls.protos[(addr, port)] = None
             return response
 
-        if (addr, port) not in cls.listeners:
+        if port == 0 or (addr, port) not in cls.listeners:
             # When creating a listener on a given (addr, port) we only need to
             # do it once. If we want different behaviors for different
             # connections, we can accomplish this by providing different
             # `proto` functions
 
             listener = await cls.network_event_loop.create_server(peer_protocol, addr, port)
+            port = listener.sockets[0].getsockname()[1]
             logger.debug("Listening server on %s:%d should be started" % (addr, port))
             cls.listeners[(addr, port)] = listener