This was revealed in #1114; onchaind isn't actually completely idempotent
due to fee changes (and the now-fixed change in keys used).
This triggers the bug by restarting with different fees, resulting in
onchaind not recognizing its own proposal:
2018-03-05T09:38:15.550Z lightningd(23076): lightning_onchaind-022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59 chan #1: STATUS_FAIL_INTERNAL_ERROR: THEIR_UNILATERAL/OUR_HTLC spent with weird witness 3
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>