test: re-enable CLI test support by using EncodeDecimal in json.dumps()

As mentioned in
https://github.com/bitcoin/bitcoin/pull/17675#issuecomment-563188648

test/functional/wallet_listsinceblock.py

@@ -19,7 +19,6 @@ class ListSinceBlockTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 4
         self.setup_clean_chain = True
-        self.supports_cli = False
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()