This is a side-effect of fixing aging: sometimes, we age our
rcvd_filter cache too fast, and thus re-xmit. This breaks
our test, since it used dev-disconnect on the channel_announce,
but that closes to l3, not l1!
```
> assert l1.rpc.listchannels()['channels'] == []
E AssertionError: assert [{'active': T...ags': 1, ...}] == []
E Left contains 2 more items, first extra item: {'active': True, 'amount_msat': 100000000msat, 'base_fee_millisatoshi': 1, 'channel_flags': 0, ...}
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Fixes: #5403
06e1e119aa