I'm not sure I understand what you mean by "PHPUnit" - quick google search... oh, it's some goofy framework crap to avoid installing a testing server; my advice, pitch that in the trash just like every other blasted framework. Nothing like wasting time making more work for yourself and then being told it's less work and saves time. Frameworks... pah.
Just run it as a normal script.
That said, I'm just not seeing how what you have there would even do anything since I don't know what you are calling or what it's building since again, no clue what that XML class you are using actually does or outputs. I know in normal XML parsers like simpleXML or DOMDocument in MOST cases a return like ['is_valid'] would be deeply nested, NOT in the top of any array structure.
... and without seeing the XML structure you are trying to parse, anything we try to tell you is nothing more than a wild stab in the dark. Though again, it doesn't look like the code you presented should actually work, unless that unknown XML class is doing something really goofy or strange to that file.