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 = []