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 <rusty@rustcorp.com.au>
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