test: Replace importprivkey with wallet_importprivkey

importprivkey was a legacy wallet only RPC which had a helper for
descriptor wallets in tests. Add wallet_importprivkey helper and use it
wherever importprivkey is used (other than backward compatibility tests)

test/functional/wallet_listsinceblock.py

@@ -13,6 +13,7 @@ from test_framework.util import (
     assert_array_result,
     assert_equal,
     assert_raises_rpc_error,
+    wallet_importprivkey,
 )
 from test_framework.wallet_util import generate_keypair
 
@@ -227,10 +228,10 @@ class ListSinceBlockTest(BitcoinTestFramework):
         address = key_to_p2wpkh(pubkey)
         self.nodes[2].sendtoaddress(address, 10)
         self.generate(self.nodes[2], 6)
-        self.nodes[2].importprivkey(privkey)
+        wallet_importprivkey(self.nodes[2], privkey, "now")
         utxos = self.nodes[2].listunspent()
         utxo = [u for u in utxos if u["address"] == address][0]
-        self.nodes[1].importprivkey(privkey)
+        wallet_importprivkey(self.nodes[1], privkey, "now")
 
         # Split network into two
         self.split_network()