Technically, Clinton's testimony was not subpoenaed. Ken Starr issued a subpoena, which
many people questioned whether he had the legal authority to do (and most said if he did it was only because the issue had hand had nothing to do with official actions).
The status of the Office of the Independent Counsel was a gray area that had never been tried. Much as Bush has done with several subpoenas Clinton said he would not respond to the subpoena but if it were withdrawn he would testify willingly under negotiated terms (one day only, at the White House instead of before the grand jury). The subpoena was withdrawn so the question was never settled. The the Office of the Independent Counsel was abolished and now any subpoena would be for direct testimony to a congressional committee about official actions. I doubt that would hold up if taken to court.