Fix broken ArcanistPhpunitTestResultParser

Summary:
The ArcanistPhpunitTestResultParser has been changed
to use phutil_json_decode instead of json_decode in
rARCa4d33ef117aa8702181154b60ce1ce52bcfc119b
That has broken the functionality as json_decode has returned
an object before but phutil_json_decode does return an array.
The code has now been adopted to work with the array result
instead of an object.

Test Plan: Run a phpunit test case

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: aurelijus, epriestley

Differential Revision: https://secure.phabricator.com/D12751