[gdb/testsuite] Handle unordered dict in gdb.python/py-mi-cmd.exp

[gdb/testsuite] Handle unordered dict in gdb.python/py-mi-cmd.exp

When running test-case gdb.python/py-mi-cmd.exp on openSUSE Leap 42.3 with
python 3.4, I occasionally run into:
...
Expecting: ^(-pycmd dct[^M
]+)?(\^done,result={hello="world",times="42"}[^M
]+[(]gdb[)] ^M
[ ]*)
-pycmd dct^M
^done,result={times="42",hello="world"}^M
(gdb) ^M
FAIL: gdb.python/py-mi-cmd.exp: -pycmd dct (unexpected output)
...

The problem is that the data type used here in py-mi-cmd.py:
...
        elif argv[0] == "dct":
            return {"result": {"hello": "world", "times": 42}}
...
is a dictionary, and only starting version 3.6 are dictionaries insertion
ordered, so using PyDict_Next in serialize_mi_result doesn't guarantee a
fixed order.

Fix this by allowing the alternative order.

Tested on x86_64-linux.