test: wallet_backup.py, fix intermittent failure in "restore using dumped wallet"

The failure arises because the test expects 'init_wallet()' (the test
framework function) creating 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 behind the intermittent failures might be other peers delivering
the chain right after node2 startup (sync of the validation queue included)
and prior to the 'node2.getbalance()' check.

test/functional/wallet_backup.py

@@ -140,11 +140,6 @@ class WalletBackupTest(BitcoinTestFramework):
         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 @@ class WalletBackupTest(BitcoinTestFramework):
             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)