If this behavior has appeared suddenly, it is likely a reflection of recent stressors, and I would focus on what the stressor(s) might be. On the other hand, if the behavior is more chronic in nature, it might lead more in the direction of a possible mood disorder. What argues against this is her high degree of functioning out side the home setting. Early adolescents by nature can prove difficult in their interactions with parents, even when the parent is doing the best he/she can. I'd lean in the direction of focusing on the arena of family interactions and see if some adjustments can be made. Perhaps the therapist can be useful in this regard. I don't see the need for testing.
Lisa