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_balance.py

@@ -54,7 +54,6 @@ class WalletTest(BitcoinTestFramework):
             ['-limitdescendantcount=3'],  # Limit mempool descendants as a hack to have wallet txs rejected from the mempool
             [],
         ]
-        self.supports_cli = False
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()