Where does the anxiety come from? Is it perfectionism? Discomfort with challenge? I think helping her discover what it is that goes through her mind and identifying strategies to work past and desensitize her to them is the way.
If it is the challenge, maybe some online testing above her level where she might only know a quarter of the material could get her used to that.
If it is just timing, then online math games where you get more points for speed.
For any source having a test taking strategy can be helpful. Like doing a first pass where she does only the obvious ones, then a second pass for slightly harder, and such. Also, having an inner voice coach herself is a skill to develop; positive self talk, rational objective point of view, etc. Meditation is a way to discover that. Visualization exercises focused on test taking.
