XFAIL : tst_QScriptEngine::installTranslatorFunctions(Custom global object) FIXME: why we expect that String prototype exists?

Setting a custom global object should not impact what prototype is used for String objects. ECMA-262 15.5.2.1: "The [[Prototype]] internal property of the newly constructed object is set to the standard built-in String prototype object that is the initial value of String.prototype". And that's the object where we added the "arg" property.