test: make reusable SOCKS5 server starting

Extract the part of `p2p_private_broadcast.py` that configures and
starts the SOCKS5 server into a reusable function and put it into
`test_framework/socks5.py`.

Use bind port 0 to let the OS pick an available port instead of
hackishly assuming that `p2p_port(N)` is available where N is more
than the number of the nodes the test uses.

Github-Pull: #35410
Rebased-From: 2ffa81fac4

test/functional/test_framework/socks5.py

@@ -327,3 +327,15 @@ class Socks5Server():
                     logger.debug(f"Stop(): Handler {i} thread joined")
                 else:
                     logger.warning(f"Stop(): Handler thread {i} didn't finish after force close")
+
+def start_socks5_server(destinations_factory):
+    config = Socks5Configuration()
+    config.addr = ("127.0.0.1", 0) # Use port=0 to let the OS pick one. The actual port is later in server.conf.addr[1].
+    config.unauth = True
+    config.auth = True
+    config.destinations_factory = destinations_factory
+
+    server = Socks5Server(config)
+    server.start()
+
+    return server