pytest: fix flake in test_splicing_rbf

We get a unilateral close if the blocks come too fast:

```
>       l2.daemon.wait_for_log(r'CHANNELD_AWAITING_SPLICE to CHANNELD_NORMAL')

tests/test_splicing.py:96:
...
>                   raise TimeoutError('Unable to find "{}" in logs.'.format(exs))
E                   TimeoutError: Unable to find "[re.compile('CHANNELD_AWAITING_SPLICE to CHANNELD_NORMAL')]" in logs.
```

Here:

```
lightningd-2 2025-07-08T12:08:30.430Z DEBUG   lightningd: Adding block 114: 542499dfc63bc94c0ba0a3d81bdaff07ab73ba9aebddd4c2ae4b2e0c1e8bd15f
...
lightningd-2 2025-07-08T12:08:30.459Z UNUSUAL 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#1: Peer permanent failure in CHANNELD_AWAITING_SPLICE: Fulfilled HTLC 1 RCVD_REMOVE_REVOCATION cltv11 4 hit deadline (reason=protocol)
lightningd-2 2025-07-08T12:08:30.469Z DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#1: Status closed, but not exited. Killing
lightningd-2 2025-07-08T12:08:30.475Z INFO    0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#1: State changed from CHANNELD_AWAITING_SPLICE to AWAITING_UNILATERAL
```

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

Author: rustyrussell

tests/test_splicing.py

@@ -91,6 +91,9 @@ def test_splice_rbf(node_factory, bitcoind):
     inv = l2.rpc.invoice(10**2, '2', 'no_2')
     l1.rpc.pay(inv['bolt11'])
 
+    # Make sure l1 doesn't unilateral close if HTLC hasn't completely settled before deadline.
+    wait_for(lambda: only_one(l1.rpc.listpeerchannels()['channels'])['htlcs'] == [])
+
     bitcoind.generate_block(6, wait_for_mempool=1)
 
     l2.daemon.wait_for_log(r'CHANNELD_AWAITING_SPLICE to CHANNELD_NORMAL')