pytest: add debug print to diagnose flake in test_peer_anchor_push

Sometimes the feerate is 5 too high.  But without seeing the actual txs it's
looking at, it's hard to know why.

```
actual_feerate = 13005.66942869603, expected_feerate = 13000

    def check_feerate(nodes, actual_feerate, expected_feerate):
        # Feerate can't be lower.
        assert actual_feerate > expected_feerate - 2
        if actual_feerate >= expected_feerate + 2:
            if any([did_short_sig(n) for n in nodes]):
                return
        # Use assert as it shows the actual values on failure
>       assert actual_feerate < expected_feerate + 2
E       AssertionError
```

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

Author: rustyrussell

tests/test_closing.py

@@ -4056,6 +4056,7 @@ def test_peer_anchor_push(node_factory, bitcoind, executor, chainparams):
         l2.daemon.wait_for_log("sendrawtx exit 0")
         # Check feerate for entire package (commitment tx + anchor) is ~ correct
         details = bitcoind.rpc.getrawmempool(True).values()
+        print(f"mempool = {details}")
         total_weight = sum([d['weight'] for d in details])
         total_fees = sum([float(d['fees']['base']) * 100_000_000 for d in details])
         total_feerate_perkw = total_fees / total_weight * 1000