test: Fix backwards compatibility intermittent failure

Author: aureleoules

test/functional/feature_backwards_compatibility.py

@@ -274,6 +274,7 @@ def run_test(self):
             assert_equal(info["desc"], descsum_create(descriptor))
 
             # Now copy that same wallet back to 0.16 to make sure no automatic upgrade breaks it
+            node_master.unloadwallet("u1_v16")
             os.remove(os.path.join(node_v16_wallets_dir, "wallets/u1_v16"))
             shutil.copyfile(
                 os.path.join(node_master_wallets_dir, "u1_v16"),