To my mind and I’d dare say an honest prosecutor, judge and jury’s mind, an opening a statement that “I did nothing wrong, illegal or otherwise” is Testimony. Further invocations of protecting oneself from self-incrimination are of lesser standing.
Whether she answered questions or not, she was “on the stand” and her statements should be used against her at trial.
Oh! Oh! Oh! and BTW, presented with the same situation in the Watergate hearings, young Ms Hillary (a staffer on the prosecution side) vehemently urged the defendants (accused) be stripped of their constitutional rights, including that of which Lerner availed herself.