pytest: fix flake in test_xpay_twohop_bug

CLTV can be one less if l1 hasn't seen block yet:

```
2025-04-24T05:38:12.8587408Z >       l1.daemon.wait_for_log(f'Adding HTLC 0 amount=15002msat cltv={110 + 1 + 100 + 200 + 400}')
2025-04-24T05:38:12.8587715Z 
2025-04-24T05:38:12.8587804Z tests/test_xpay.py:824: 
...
2025-04-24T05:38:12.9889506Z lightningd-1 2025-04-24T05:30:45.255Z DEBUG   022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-channeld-chan#1: Adding HTLC 0 amount=15002msat cltv=812 gave CHANNEL_ERR_ADD_OK
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

tests/test_xpay.py

@@ -3,7 +3,8 @@
 from pyln.client import RpcError
 from pyln.testing.utils import FUNDAMOUNT, only_one
 from utils import (
-    TIMEOUT, first_scid, GenChannel, generate_gossip_store, wait_for
+    TIMEOUT, first_scid, GenChannel, generate_gossip_store, wait_for,
+    sync_blockheight,
 )
 
 import os
@@ -776,7 +777,7 @@ def test_fail_after_success(node_factory, bitcoind, executor, slow_mode):
                        'successful_parts': 1}
 
 
-def test_xpay_twohop_bug(node_factory):
+def test_xpay_twohop_bug(node_factory, bitcoind):
     """From https://github.com/ElementsProject/lightning/issues/8119:
     Oh, interesting! I tried again and got a two-hop blinded path. xpay returned the same error you saw while pay was successful.
 
@@ -818,6 +819,9 @@ def test_xpay_twohop_bug(node_factory):
     assert len(path['path']) == 2
     assert path['payinfo']['cltv_expiry_delta'] == 200 + 400
 
+    # Make sure l1 is on correct height, so CLTV is as expected.
+    sync_blockheight(bitcoind, [l1])
+
     # This works.
     l1.rpc.pay(inv)
     # CLTV is blockheight (110) + 1 + 100 + 200 + 400