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/mempool_packages.py

@@ -27,7 +27,6 @@ class MempoolPackagesTest(BitcoinTestFramework):
             ["-maxorphantx=1000"],
             ["-maxorphantx=1000", "-limitancestorcount={}".format(MAX_ANCESTORS_CUSTOM)],
         ]
-        self.supports_cli = False
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()