If we fund a channel between two nodes, then mine all the blocks to
announce it, any other nodes may see the announcement before the
blocks, causing CI to complain about "bad gossip":
```
lightningd-4: 2022-01-25T22:33:25.468Z DEBUG 032cf15d1ad9c4a08d26eab1918f732d8ef8fdc6abb9640bf3db174372c491304e-gossipd: Ignoring future channel_announcment for 113x1x1 (current block 112)
lightningd-4: 2022-01-25T22:33:25.468Z DEBUG 032cf15d1ad9c4a08d26eab1918f732d8ef8fdc6abb9640bf3db174372c491304e-gossipd: Bad gossip order: WIRE_CHANNEL_UPDATE before announcement 113x1x1/0
lightningd-4: 2022-01-25T22:33:25.468Z DEBUG 032cf15d1ad9c4a08d26eab1918f732d8ef8fdc6abb9640bf3db174372c491304e-gossipd: Bad gossip order: WIRE_CHANNEL_UPDATE before announcement 113x1x1/1
lightningd-4: 2022-01-25T22:33:25.468Z DEBUG 032cf15d1ad9c4a08d26eab1918f732d8ef8fdc6abb9640bf3db174372c491304e-gossipd: Bad gossip order: WIRE_NODE_ANNOUNCEMENT before announcement 032cf15d1ad9c4a08d26eab1918f732d8ef8fdc6abb9640bf3db174372c491304e
```
Add a new helper for this case, and use it where there are more than 2 nodes.
Cleans up test_routing_gossip and a few other places which did this manually.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>