Former Navy Lieut. Paula Coughlin's lawsuit against the Las Vegas Hilton--where she claims she was groped during the 1991 Tailhook convention--went to trial today. She unexpectedly settled with Tailhook, an organization of naval aviators, last Friday; but the deal with the hotel fell through. She is seeking an unspecified sum from the Hilton Hotel Corp. for negligence. Jury selection began today in a court case that is expected to last five weeks.