pytest: fix flake in test_channel_lease_unilat_closes

We need to make sure the lease starts a the block we expect:

```
        # Note that l3 has the whole lease delay (minus blocks already mined)
        _, _, l3blocks = l3.wait_for_onchaind_tx('OUR_DELAYED_RETURN_TO_WALLET',
                                                 'OUR_UNILATERAL/DELAYED_OUTPUT_TO_US')
>       assert l3blocks == 4032 - 6 - 2 - 1
E       assert 4022 == (((4032 - 6) - 2) - 1)

tests/test_closing.py:985: AssertionError
```

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

Author: rustyrussell

tests/test_closing.py

@@ -932,6 +932,9 @@ def test_channel_lease_unilat_closes(node_factory, bitcoind):
     l2.fundwallet(20000000)
     l3.fundwallet(20000000)
 
+    # Make sure we are at known height for lease.
+    sync_blockheight(bitcoind, [l1, l2, l3])
+
     l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
     rates = l1.rpc.dev_queryrates(l2.info['id'], amount, amount)
     # l1 leases a channel from l2