* Gaddafi sons' trial seen as test for Libyan state
* Saif al-Islam being held by former Zintan rebels
* Human rights groups concerned over trial terms (Recasts with start of trial procedures)
By Julia Payne
TRIPOLI, April 14 (Reuters) - Libyan prosecutors opened the trial of deposed leader Muammar Gaddafi's sons and former regime officials on Monday in a major test for the North ...