pytest: fix flake in splice gossip test.

We can in fact see the new channel before this line is called:

```
2025-03-15T12:31:04.1472196Z     @pytest.mark.openchannel('v1')
2025-03-15T12:31:04.1472616Z     @pytest.mark.openchannel('v2')
2025-03-15T12:31:04.1473317Z     @unittest.skipIf(TEST_NETWORK != 'regtest', 'elementsd doesnt yet support PSBT features we need')
2025-03-15T12:31:04.1474271Z     def test_splice_gossip(node_factory, bitcoind):
2025-03-15T12:31:04.1475078Z         l1, l2, l3 = node_factory.line_graph(3, fundamount=1000000, wait_for_announce=True, opts={'experimental-splicing': None})
2025-03-15T12:31:04.1475781Z
2025-03-15T12:31:04.1476052Z         chan_id = l1.get_channel_id(l2)
2025-03-15T12:31:04.1476460Z         pre_splice_scid = first_scid(l1, l2)
2025-03-15T12:31:04.1476844Z
2025-03-15T12:31:04.1477134Z         # add extra sats to pay fee
2025-03-15T12:31:04.1477741Z         funds_result = l1.rpc.fundpsbt("109000sat", "slow", 166, excess_as_change=True)
2025-03-15T12:31:04.1478345Z
2025-03-15T12:31:04.1478765Z         result = l1.rpc.splice_init(chan_id, 100000, funds_result['psbt'])
2025-03-15T12:31:04.1479432Z         result = l1.rpc.splice_update(chan_id, result['psbt'])
2025-03-15T12:31:04.1479994Z         assert(result['commitments_secured'] is False)
2025-03-15T12:31:04.1480584Z         result = l1.rpc.splice_update(chan_id, result['psbt'])
2025-03-15T12:31:04.1481089Z         assert(result['commitments_secured'] is True)
2025-03-15T12:31:04.1481386Z         result = l1.rpc.signpsbt(result['psbt'])
2025-03-15T12:31:04.1481860Z         result = l1.rpc.splice_signed(chan_id, result['signed_psbt'])
2025-03-15T12:31:04.1482403Z
2025-03-15T12:31:04.1485960Z         wait_for(lambda: only_one(l2.rpc.listpeerchannels(l1.info['id'])['channels'])['state'] == 'CHANNELD_AWAITING_SPLICE')
2025-03-15T12:31:04.1489978Z         wait_for(lambda: only_one(l1.rpc.listpeerchannels(l2.info['id'])['channels'])['state'] == 'CHANNELD_AWAITING_SPLICE')
2025-03-15T12:31:04.1490796Z
2025-03-15T12:31:04.1491223Z         bitcoind.generate_block(6, wait_for_mempool=result['txid'])
2025-03-15T12:31:04.1491767Z
2025-03-15T12:31:04.1492213Z         # l3 will see channel dying, but still consider it OK for 12 blocks.
2025-03-15T12:31:04.1493174Z         l3.daemon.wait_for_log(f'gossipd: channel {pre_splice_scid} closing soon due to the funding outpoint being spent')
2025-03-15T12:31:04.1494422Z         assert len(l3.rpc.listchannels(short_channel_id=pre_splice_scid)['channels']) == 2
2025-03-15T12:31:04.1495293Z >       assert len(l3.rpc.listchannels(source=l1.info['id'])['channels']) == 1
2025-03-15T12:31:04.1495937Z E       AssertionError: assert 2 == 1
2025-03-15T12:31:04.1503185Z E        +  where 2 = len([{'active': True, 'amount_msat': 1000000000, 'base_fee_millisatoshi': 1, 'channel_flags': 1, ...}, {'active': True, 'amount_msat': 1100000000, 'base_fee_millisatoshi': 1, 'channel_flags': 1, ...}])
```

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

Author: rustyrussell

tests/test_splicing.py

@@ -96,13 +96,15 @@ def test_splice_gossip(node_factory, bitcoind):
     wait_for(lambda: only_one(l2.rpc.listpeerchannels(l1.info['id'])['channels'])['state'] == 'CHANNELD_AWAITING_SPLICE')
     wait_for(lambda: only_one(l1.rpc.listpeerchannels(l2.info['id'])['channels'])['state'] == 'CHANNELD_AWAITING_SPLICE')
 
-    bitcoind.generate_block(6, wait_for_mempool=result['txid'])
+    bitcoind.generate_block(5, wait_for_mempool=result['txid'])
 
     # l3 will see channel dying, but still consider it OK for 12 blocks.
     l3.daemon.wait_for_log(f'gossipd: channel {pre_splice_scid} closing soon due to the funding outpoint being spent')
     assert len(l3.rpc.listchannels(short_channel_id=pre_splice_scid)['channels']) == 2
     assert len(l3.rpc.listchannels(source=l1.info['id'])['channels']) == 1
 
+    # Final one will allow splice announcement to proceed.
+    bitcoind.generate_block(1)
     wait_for(lambda: only_one(l2.rpc.listpeerchannels(l1.info['id'])['channels'])['state'] == 'CHANNELD_NORMAL')
     wait_for(lambda: only_one(l1.rpc.listpeerchannels(l2.info['id'])['channels'])['state'] == 'CHANNELD_NORMAL')