We fail waiting for 'Resolved OUR_UNILATERAL/DELAYED_OUTPUT_TO_US by our proposal OUR_DELAYED_RETURN_TO_WALLET'
because we close *two* channels, but only wait for one transaction before mining a block.
This means we might only have one, and we immediately mine the next wait_for_mempool=1,
so the OUR_DELAYED_RETURN_TO_WALLET isn't mined.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>