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/test_framework/util.py

@@ -190,6 +190,11 @@ def check_json_precision():
     if satoshis != 2000000000000003:
         raise RuntimeError("JSON encode/decode loses precision")
 
+def EncodeDecimal(o):
+    if isinstance(o, Decimal):
+        return str(o)
+    raise TypeError(repr(o) + " is not JSON serializable")
+
 def count_bytes(hex_string):
     return len(bytearray.fromhex(hex_string))