Various formulations of the controlling test have been used to determine whether institutions or individuals are immune: whether they 'have been so incorporated into the government structure as to become instrumentalities of the United States and thus emjoy governmental immunity,' United States v. Boyd, 378 U.S. 39, 48, 84 S.Ct. 1518, 1524, 12 L.Ed.2d 713 (1964); whether they 'are arms of the gove