I actually hit this very hard to reproduce race: if we haven't process
the channeld message when block #6 comes in, we won't send the gossip
message. We wait for logs, but don't generate new blocks, and timeout
on l1.daemon.wait_for_log('peer_out WIRE_ANNOUNCEMENT_SIGNATURES').
The solution, which also tests that we don't send announcement signatures
immediately, is to generate a single block, wait for CHANNELD_NORMAL,
then (in gossip tests), generate 5 more.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>