It's plainly wrong for a member of Congress to collaborate with a public relations firm to produce knowingly deceptive testimony on an important issue. Yet Representative Tom Lantos of California has been caught doing exactly that. Mr. Lantos is co-chairman of the Congressional Human Rights Caucus. An article last week on The Times's Op-Ed page by John MacArthur, the publisher of Harper's magazine, revealed the identity of a 15-year-old Kuwaiti girl who told the caucus that Iraqi soldiers had removed scores of babies from incubators and left them to die. The girl, whose testimony helped build support for the Persian Gulf war, was identified only as Nayirah." She is not just some Kuwaiti but the daughter of the Kuwaiti Ambassador to the U.S.. Saddam Hussein committed plenty of atrocities, but not, apparently, this one. The teen-ager's accusation, at first verified by Amnesty International, was later refuted by that group as well as by other independent human rights monitors. But the issue is not so much the accuracy of the testimony as the identity and undisclosed bias of the witness. How did the girl's testimony come about? It was arranged by the big public relations firm of Hill & Knowlton on behalf of a client, the Kuwaiti-sponsored Citizens for a Free Kuwait, which was then pressing Congress for military intervention. Mr. Lantos knew the girl's identity but concealed it from the public and from the other caucus co-chairman, Representative John E. Porter of Illinois.