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/test_framework/util.py

@@ -20,6 +20,7 @@ import time
 
 from . import coverage
 from .authproxy import AuthServiceProxy, JSONRPCException
+from .descriptors import descsum_create
 from collections.abc import Callable
 from typing import Optional, Union
 
@@ -609,3 +610,13 @@ def sync_txindex(test_framework, node):
     sync_start = int(time.time())
     test_framework.wait_until(lambda: node.getindexinfo("txindex")["txindex"]["synced"])
     test_framework.log.debug(f"Synced in {time.time() - sync_start} seconds")
+
+def wallet_importprivkey(wallet_rpc, privkey, timestamp, *, label=""):
+    desc = descsum_create("combo(" + privkey + ")")
+    req = [{
+        "desc": desc,
+        "timestamp": timestamp,
+        "label": label,
+    }]
+    import_res = wallet_rpc.importdescriptors(req)
+    assert_equal(import_res[0]["success"], True)