[tests] Set -deprecatedrpc=accounts in tests

Future commits will deprecate the accounts RPC methods, arguments and
return objects. Set the -deprecatedrpc=accounts switch now so tests
don't break in intermediate commits.

test/functional/wallet_txn_doublespend.py

@@ -10,6 +10,7 @@ from test_framework.util import *
 class TxnMallTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 4
+        self.extra_args = [['-deprecatedrpc=accounts']] * 4
 
     def add_options(self, parser):
         parser.add_option("--mineblock", dest="mine_block", default=False, action="store_true",