qa: Use node.datadir instead of tmpdir in test framework

test/functional/wallet_multiwallet.py

@@ -10,7 +10,11 @@ import os
 import shutil
 
 from test_framework.test_framework import BitcoinTestFramework
-from test_framework.util import assert_equal, assert_raises_rpc_error
+from test_framework.util import (
+    assert_equal,
+    assert_raises_rpc_error,
+)
+
 
 class MultiWalletTest(BitcoinTestFramework):
     def set_test_params(self):