BURLINGTON, Vt. - Vowing to fight the introduction of the abortion pill RU-486 into the U.S. drug market, two national pro-life leaders visited the Vermont city chosen as a test site for the drug. Father Frank Pavone of the New York-based Priests for Life and the Rev. Philip "Flip" Benham of the national Operation Rescue organization addressed a pro-life rally at Our Lady of Grace Parish in Colchester Dec. 2 as part of a weekend of protests against the Burlington-area testing of RU-486. The leaders urged their audience to step up their pro-life activities and stop RU-486 from being approved for use in the United States by the Food and Drug