test: convert tuple to json for cli

This makes it possible to run rpc_createmultisig.py with --usecli.

test/functional/rpc_createmultisig.py

@@ -28,7 +28,6 @@ class RpcCreateMultiSigTest(BitcoinTestFramework):
     def set_test_params(self):
         self.setup_clean_chain = True
         self.num_nodes = 3
-        self.supports_cli = False
 
     def create_keys(self, num_keys):
         self.pub = []

test/functional/test_framework/test_node.py

@@ -881,7 +881,7 @@ def arg_to_cli(arg):
         return str(arg).lower()
     elif arg is None:
         return 'null'
-    elif isinstance(arg, dict) or isinstance(arg, list):
+    elif isinstance(arg, dict) or isinstance(arg, list) or isinstance(arg, tuple):
         return json.dumps(arg, default=serialization_fallback)
     else:
         return str(arg)