message: test: Fix feature_notifications.py intermittent issue
diff: @@ -166,6 +166,8 @@ class NotificationsTest(BitcoinTestFramework):
  # Should now verify contents of each file
  for tx_id, blockheight, blockhash in tx_details:
  fname = os.path.join(self.walletnotify_dir, notify_outputname(self.wallet, tx_id))
  + # Wait for the cached writes to hit storage
  + self.wait_until(lambda: os.path.getsize(fname) > 0, timeout=10)
  with open(fname, 'rt', encoding='utf-8') as f:
  text = f.read()
  # Universal newline ensures '\n' on 'nt'
  