bkpr-test: maybe fix race in test_bookkeeping_closing_trimmed_htlcs

test_bookkeeping_closing_trimmed_htlcs fails to find 'all outputs
resolved' occassionally, seems like it's because the
OUR_DELAYED_TO_WALLET doesn't make it into the mempool before we start
mining blocks?

So here make sure there's something in the mempool before before we
start making new blocks.

tests/test_bookkeeper.py

@@ -46,7 +46,7 @@ def test_bookkeeping_closing_trimmed_htlcs(node_factory, bitcoind, executor):
     bitcoind.generate_block(5)
     sync_blockheight(bitcoind, [l1])
     l1.daemon.wait_for_log('Broadcasting OUR_DELAYED_RETURN_TO_WALLET')
-    bitcoind.generate_block(20)
+    bitcoind.generate_block(20, wait_for_mempool=1)
     sync_blockheight(bitcoind, [l1])
     l1.daemon.wait_for_log(r'All outputs resolved.*')