Merge bitcoin/bitcoin#26658: test: Fix backwards compatibility intermittent failure

c29bff5 test: Fix backwards compatibility intermittent failure (Aurèle Oulès)

Pull request description:

  Fixes #24400.

  See bitcoin/bitcoin#24400 (comment) to reproduce the failure.

  As MarcoFalke suggested in bitcoin/bitcoin#24400 (comment), it can happen that the wallet is not fully flushed before being copied over to the other node. Fixed by unloading the wallet before copying the file.

ACKs for top commit:
  Sjors:
    utACK c29bff5

Tree-SHA512: ea93b859878873d07fa44ccb1c5b4c1b1014be8568dc2b9bbef02bb7a9230fefa7a71b149eb553870d3f4e986263342ea4f996fa0221098a2244f38952100471

Author: MarcoFalke

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"),