pytest: fix flake in test_channel_lease_unilat_closes

```
2026-01-30T05:55:13.6654636Z         # Note that l3 has the whole lease delay (minus blocks already mined)
2026-01-30T05:55:13.6655396Z         _, _, l3blocks = l3.wait_for_onchaind_tx('OUR_DELAYED_RETURN_TO_WALLET',
2026-01-30T05:55:13.6656086Z                                                  'OUR_UNILATERAL/DELAYED_OUTPUT_TO_US')
2026-01-30T05:55:13.6656618Z >       assert l3blocks == 4032 - 6 - 2 - 1
2026-01-30T05:55:13.6657033Z E       assert 4025 == (((4032 - 6) - 2) - 1)
```

Turns out that 4342043 (tests: de-flake test that was failing on
cltv expiry) added a line to mine two more blocks, but the hardcoded
110 was not changed to 112, so we weren't actually waiting correctly.

Remove hardcoded numbers in favor of calculation, and do the same in
test_channel_lease_post_expiry (which was correct, for now).

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

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

Author: rustyrussell

tests/test_closing.py

@@ -871,7 +871,8 @@ def test_channel_lease_post_expiry(node_factory, bitcoind, chainparams):
     bitcoind.generate_block(6)
     sync_blockheight(bitcoind, [l1, l2])
     # make sure we're at the right place for the csv lock
-    l2.daemon.wait_for_log('Blockheight: SENT_ADD_ACK_COMMIT->RCVD_ADD_ACK_REVOCATION LOCAL now 115')
+    height = bitcoind.rpc.getblockchaininfo()['blocks']
+    l2.daemon.wait_for_log(f'Blockheight: SENT_ADD_ACK_COMMIT->RCVD_ADD_ACK_REVOCATION LOCAL now {height}')
 
     # We need to give l1-l2 time to update their blockheights
     for i in range(0, 4000, 1000):
@@ -980,7 +981,8 @@ def test_channel_lease_unilat_closes(node_factory, bitcoind):
     bitcoind.generate_block(2)
     sync_blockheight(bitcoind, [l1, l2, l3])
     # make sure we're at the right place for the csv lock
-    l2.daemon.wait_for_log('Blockheight: SENT_ADD_ACK_COMMIT->RCVD_ADD_ACK_REVOCATION LOCAL now 110')
+    height = bitcoind.rpc.getblockchaininfo()['blocks']
+    l2.daemon.wait_for_log(f'Blockheight: SENT_ADD_ACK_COMMIT->RCVD_ADD_ACK_REVOCATION LOCAL now {height}')
     l2.stop()
 
     # unilateral close channels l1<->l2 & l3<->l2