First, simplify: amount is set to 1000000, but then we deposit 1000000 + 0.01btc
(i.e. 2000000), and we always use 2 * amount. Just use a single constant to
make it clear.
Secondly, we assume that the wallet considers outputs spent as soon as
we created the tx: this will not be true once withdraw uses sendpsbt.
So, we generate blocks, but now sometimes withdraw will pick up change
txs, so we need to reserve them to avoid that messing our coinmovements.
Finally, we assumed the withdrawl order was BIP69, which becomes
variable.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>