ipatests: Extend the order plugin to properly handle inheritance
    
    When trying to create a new ordered test case by inheriting
    from already defined test case, by overriding few of its methods,
    the execution order of the tests is as follows:
        - first all non-overriden test methods from the parent test class
        - then all overriden tests methods
    
    This patch makes sure that methods are executed in the logical order,
    that is, the order defined in the parent class.