[tests] move generate_wif_key to wallet_util.py

generate_wif_key is a wallet utility function. Move
it from the EC key module to the wallet util module.

test/functional/rpc_createmultisig.py

@@ -11,12 +11,13 @@ import os
 
 from test_framework.authproxy import JSONRPCException
 from test_framework.descriptors import descsum_create, drop_origins
-from test_framework.key import ECPubKey, ECKey, bytes_to_wif
+from test_framework.key import ECPubKey, ECKey
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_raises_rpc_error,
     assert_equal,
 )
+from test_framework.wallet_util import bytes_to_wif
 
 class RpcCreateMultiSigTest(BitcoinTestFramework):
     def set_test_params(self):