pytest: make test_no_delay more robust.

Unfortunately the effect of leaving Nagle enabled is subtle.  Here it
is in v25.12:

Normal: 
    tests/test_connection.py::test_no_delay PASSED
    ====================================================================== 1 passed in 13.87s

Nagle enabled:
    tests/test_connection.py::test_no_delay PASSED
    ====================================================================== 1 passed in 21.70s

So it's hard to both catch this issue and not have false positives.  Improve the
test by deliberately running with Nagle enabled, so we can do a direct comparison.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

connectd/connectd_wire.csv

@@ -28,6 +28,7 @@ msgdata,connectd_init,dev_throttle_gossip,bool,
 msgdata,connectd_init,dev_no_reconnect,bool,
 msgdata,connectd_init,dev_fast_reconnect,bool,
 msgdata,connectd_init,dev_limit_connections_inflight,bool,
+msgdata,connectd_init,dev_keep_nagle,bool,
 
 # Connectd->master, here are the addresses I bound, can announce.
 msgtype,connectd_init_reply,2100