test: Fix wallet_listsinceblock.py for BDB-only wallets

Author: hebasto

test/functional/wallet_listsinceblock.py

@@ -42,7 +42,8 @@ def run_test(self):
         self.test_double_send()
         self.double_spends_filtered()
         self.test_targetconfirmations()
-        self.test_desc()
+        if self.options.descriptors:
+            self.test_desc()
         self.test_send_to_self()
 
     def test_no_blockhash(self):