This happens if l1 doesn't get a signature, so it doesn't consider
the fulfill complete, but l2 does (and thus credits l1).
This is a trivial test, but will matter should we later correctly
account for channelmoves when onchain: in that case it will actually
hard to tell, in general, what HTLC(s) were fulfilled.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>