pytest: fix race in test_peer_anchor_push.

We have to make sure the new tx has been received by bitcoind, otherwise we
could be examining the previous:

```
>           check_feerate(l2, total_feerate_perkw, feerate)

tests/test_closing.py:4053: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

node = <fixtures.LightningNode object at 0x7f645789ee90>
actual_feerate = 14000.0, expected_feerate = 15000

    def check_feerate(node, actual_feerate, expected_feerate):
        # Feerate can't be lower.
>       assert actual_feerate > expected_feerate - 2
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

tests/test_closing.py

@@ -4045,6 +4045,7 @@ def test_peer_anchor_push(node_factory, bitcoind, executor, chainparams):
     # As blocks pass, we will use anchor to boost l3's tx.
     for block, feerate in zip(range(120, 124), (12000, 13000, 14000, 15000)):
         l2.daemon.wait_for_log(fr"Worth fee [0-9]*sat for remote commit tx to get 100000000msat at block 125 \(\+{125 - block}\) at feerate {feerate}perkw")
+        l2.daemon.wait_for_log("sendrawtx exit 0")
         # Check feerate for entire package (commitment tx + anchor) is ~ correct
         details = bitcoind.rpc.getrawmempool(True).values()
         total_weight = sum([d['weight'] for d in details])