Avoid dumpprivkey in wallet_listsinceblock.py

Generate a privkey in the test framework instead of using dumpprivkey so
that descriptor wallets work in this test.

test/functional/wallet_listsinceblock.py

@@ -4,13 +4,16 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test the listsinceblock RPC."""
 
+from test_framework.address import key_to_p2wpkh
+from test_framework.key import ECKey
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.messages import BIP125_SEQUENCE_NUMBER
 from test_framework.util import (
     assert_array_result,
     assert_equal,
     assert_raises_rpc_error,
 )
+from test_framework.wallet_util import bytes_to_wif
 
 from decimal import Decimal
 
@@ -181,15 +184,21 @@ def test_double_spend(self):
 
         self.sync_all()
 
-        # Split network into two
-        self.split_network()
-
         # share utxo between nodes[1] and nodes[2]
+        eckey = ECKey()
+        eckey.generate()
+        privkey = bytes_to_wif(eckey.get_bytes())
+        address = key_to_p2wpkh(eckey.get_pubkey().get_bytes())
+        self.nodes[2].sendtoaddress(address, 10)
+        self.nodes[2].generate(6)
+        self.nodes[2].importprivkey(privkey)
         utxos = self.nodes[2].listunspent()
-        utxo = utxos[0]
-        privkey = self.nodes[2].dumpprivkey(utxo['address'])
+        utxo = [u for u in utxos if u["address"] == address][0]
         self.nodes[1].importprivkey(privkey)
 
+        # Split network into two
+        self.split_network()
+
         # send from nodes[1] using utxo to nodes[0]
         change = '%.8f' % (float(utxo['amount']) - 1.0003)
         recipient_dict = {