test: Allow pathlib.Path as RPC argument via authproxy

Also, add datadir_path property to TestNode

test/functional/test_framework/util.py

@@ -211,12 +211,6 @@ def check_json_precision():
         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))