test: extend the SOCKS5 Python proxy to actually connect to a destination

If requested, make the SOCKS5 Python proxy redirect each connection to a
given destination. Actually act as a real proxy, connecting the
client to a destination, except that the destination is not what the
client asked for.

This would enable us to "connect" to Tor addresses from the functional
tests.

test/functional/test_framework/netutil.py

@@ -167,3 +167,10 @@ def test_unix_socket():
         return False
     else:
         return True
+
+def format_addr_port(addr, port):
+    '''Return either "addr:port" or "[addr]:port" based on whether addr looks like an IPv6 address.'''
+    if ":" in addr:
+        return f"[{addr}]:{port}"
+    else:
+        return f"{addr}:{port}"