pytest: fix flake in test_listforwards_and_listhtlcs

We need to make sure the close tx is generated where we expect, so the following code
doesn't wait foreverL

```
2025-05-08T04:31:31.1071194Z >       assert l2.rpc.wait('htlcs', 'deleted', 3)['deleted'] == 5
2025-05-08T04:31:31.1071518Z 
2025-05-08T04:31:31.1071645Z tests/test_misc.py:3398: 
...
2025-05-08T04:31:31.1079841Z E       Failed: Timeout >1800.0s
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

tests/test_misc.py

@@ -3382,7 +3382,7 @@ def test_listforwards_and_listhtlcs(node_factory, bitcoind):
     assert l2.rpc.wait('htlcs', 'deleted', 0)['deleted'] == 0
 
     # 99 blocks is not enough for them to be deleted.
-    bitcoind.generate_block(97)
+    bitcoind.generate_block(97, wait_for_mempool=1)
     assert l2.rpc.wait('htlcs', 'deleted', 0)['deleted'] == 0
 
     # This will forget c23