message: Add a sync_blocks() in walletbackup.py
  Need a syncronization point after generating a block.
diff: @@ -79,6 +79,7 @@ class WalletBackupTest(BitcoinTestFramework):
  # Must sync mempools before mining.
  sync_mempools(self.nodes)
  self.nodes[3].generate(1)
  + sync_blocks(self.nodes)
  # As above, this mirrors the original bash test.
  def start_three(self):
  