Merge bitcoin/bitcoin#28325: test: wallet_backup.py, fix intermittent failure in "restore using dumped wallet"

c4929cf test: wallet_backup.py, fix intermittent failure in "restore using dumped wallet" (furszy)

Pull request description:

  Aiming to fix #25652.

  The failure arises because the test expects `init_wallet()` (the test framework function) to create a wallet with no keys. However, the function also imports the deterministic private key used to receive the coinbase coins.

  This causes a race within the "restore using dumped wallet" case, where we intend to have a new wallet (with no existing keys) to test the 'importwallet()' RPC result.
  The reason why this failure is intermittent is that it depends on other peers delivering the chain right after node2 startup and prior to the test 'node2.getbalance()' call and also the synchronization of the validation queue.

ACKs for top commit:
  MarcoFalke:
    lgtm ACK c4929cf

Tree-SHA512: 80faa590439305576086a7d6e328f2550c97b218771fc5eba0567feff78732a2605d028a30a368d50944ae3d25fdbd6d321fb97321791a356416f2b790999613

Author: fanquake

test/functional/wallet_backup.py

@@ -140,11 +140,6 @@ def restore_wallet_existent_name(self):
         assert_raises_rpc_error(-36, error_message, node.restorewallet, wallet_name, backup_file)
         assert os.path.exists(wallet_file)
 
-    def init_three(self):
-        self.init_wallet(node=0)
-        self.init_wallet(node=1)
-        self.init_wallet(node=2)
-
     def run_test(self):
         self.log.info("Generating initial blockchain")
         self.generate(self.nodes[0], 1)
@@ -230,7 +225,10 @@ def run_test(self):
             shutil.rmtree(os.path.join(self.nodes[2].chain_path, 'chainstate'))
 
             self.start_three(["-nowallet"])
-            self.init_three()
+            # Create new wallets for the three nodes.
+            # We will use this empty wallets to test the 'importwallet()' RPC command below.
+            for node_num in range(3):
+                self.nodes[node_num].createwallet(wallet_name=self.default_wallet_name, descriptors=self.options.descriptors, load_on_startup=True)
 
             assert_equal(self.nodes[0].getbalance(), 0)
             assert_equal(self.nodes[1].getbalance(), 0)