Merge bitcoin/bitcoin#27318: test: wallet_create_tx.py fix race

8aab515 test: wallet_create_tx.py fix race (furszy)

Pull request description:

  Fixes #27316

  Because wallets are internally synchronized through the validation interface,
  and the interface dispatches events on a worker thread, it is possible for a
  transaction created by the first wallet to not arrive to the second wallet
  before the second wallet attempts to use one of its outputs. This is because
  we do not wait for the `BroadcastTransaction` callback during the wallet's
  "submit to mempool" process. To address this in the tests, we need to
  manually sync the validation queue.

ACKs for top commit:
  josibake:
    ACK bitcoin/bitcoin@8aab515
  theStack:
    ACK 8aab515

Tree-SHA512: 76364370ab292a5c3ea1ed61cd353fc626a9e9cd6ce18464c24da1b3dcb34b65006e2bc42b84bbd25af03f9449231990bf789504728972db3217b569099eb309

test/functional/wallet_create_tx.py

@@ -92,6 +92,7 @@ def test_create_too_long_mempool_chain(self):
         txid = tx_data['txid']
         vout = 1
 
+        self.nodes[0].syncwithvalidationinterfacequeue()
         options = {"change_position": 0, "add_inputs": False}
         for i in range(1, 25):
             options['inputs'] = [{'txid': txid, 'vout': vout}]