Stressing the prosecution's "good faith" in safeguarding the rights of Oliver L. North and three co-defendants, a federal judge says it's time for the Iran-Contra cases to go to trial. U.S. District Judge Gerhard A. Gesell on Thursday cleared the way for the four separate trials he has ordered by rejecting defendants' claims that prosecutors violated their constitutional rights. Gesell still must rule on defense motions that unprecedented volumes of classified material will make trials impossible. But his ruling left virtually no doubt that he plans to proceed. Independent counsel Lawrence E. Walsh has chosen North, the fired National Security Council staffer who directed the Iran-Contra affair, to be tried first. Gesell is expected to set a trial date in the next few weeks.