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/rpc_txoutproof.py

@@ -14,7 +14,6 @@ class MerkleBlockTest(BitcoinTestFramework):
         self.setup_clean_chain = True
         # Nodes 0/1 are "wallet" nodes, Nodes 2/3 are used for testing
         self.extra_args = [[], [], [], ["-txindex"]]
-        self.supports_cli = False
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()