Make ArrayHelperTest resilient against undefined sort order

The documentation of usort (http://us1.php.net/manual/en/function.usort.php)
makes it clear that "If two members compare as equal, their relative order in the sorted array is undefined."
Merge pull request #314 from elgenie/master

Make ArrayHelperTest resilient against undefined sort order
Merge remote-tracking branch 'joomla/master' into staging